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Maintenance and Inspection
Cylinder and Exhaust Valve Cleaning Tips
Top End Assembly Tips
Big Bore Kits
10 Tips for rebuilding a Two-Stroke Top End
Top End FAQs
Two-Stroke Exhaust Valves Tips and Tuning
Top-end rebuilding is the most frequent and costly service routine
on two-stroke dirt bikes. Every year, dirt bike riders waste loads
of money on top-end parts that didn’t need to be replaced, or make
costly mistakes while performing repairs. This section will give you
the dos and don’ts to easy top-end rebuilding, plus some tips that
aren’t printed in your factory service manual.
Before You Start
Thoroughly wash your bike because dirt stuck to the underside of the
top frame tube could break loose when servicing and fall into the
Use a stiff plastic brush and hot soapy water to clean off the grit
and grime around the base of the cylinder, on the carburetor and
intake boot, and especially underneath the top frame rail. Degreaser
can be used on metal surfaces, but take care not to leave it on
rubber or gasket surfaces.
You’ll need at least some 3/8-inch-drive metric sockets and box
wrenches (open-end wrenches will round off the edges on the cylinder
or head nuts, and shouldn’t be used for top-end rebuilding), a
needle-nose pliers for removing circlips, and a gasket tool to
scrape the old gaskets away.
For soft tools, get some shop towels, aerosol oven cleaner, a
Scotch-Brite pad, a locking agent such as Loctite, a gasket scraper,
a brush, and a bucket of soapy water. Regarding measuring tools,
you'll need a compression tester, a feeler gauge, and a digital
A compression tester is a useful diagnostic tool, and readily
available from Sears or auto parts stores. Buy the threaded type and
make sure the kit comes with an adapter that matches the spark plug
threads of your engine. Performing a compression test is simple.
Start by removing the spark plug, thread in the adapter, and hold
the throttle wide open and the kill button on. This will prevent any
spark and enable the engine to draw in maximum airflow.
Then kick-start the engine several times until the needle on the
pressure gauge peaks. The pressure reading depends on two main
factors; the compression ratio and the altitude at which the engine
is being tested. The compression ratio will also depend on if the
engine is equipped with exhaust valves and their condition. When the
exhaust valves are in the closed position the compression ratio will
be greater than if the valves are carbon-seized in the open
The difference may yield a pressure reading 25 psi. The quality of
compression testers varies greatly.
The main thing that a compression tester can identify is a change in
condition. Whenever you rebuild the top end, take a compression
pressure reading and mark it down. When the pressure changes 20%
check the condition of the piston and rings. Pistons usually last
twice as long as rings.
Crankcase Pressure Testing
The crankcase of a two-stroke engine is sealed from the tranny. It's
important that the two crankshaft seals be in optimum condition. One
side of the crankshaft uses a dry seal and the other a wet seal. The
dry seal runs on the magneto side and the wet seal runs in oil on
the tranny side. When the dry seal wears, the crankcase sucks in hot
air, causing the mixture to run lean and overheat the engine. When
the wet seal wears, the crankcase sucks in tranny oil, causing the
engine run rich and eventually wet-foul the spark plug.
A crankcase pressure test involves the use of a vacuum pump with
spark plug adapter, and rubber plugs to block off the intake and
exhaust manifolds of the cylinder. The piston must be positioned at
BDC to allow the transfer ports to be wide open linking the bore and
the crankcase. The hand-pump produces vacuum pressure up to a
standard setting of 5 psi. The normal bleed-down pressure loss is 1
psi per minute. Cylinders with complicated exhaust valve systems can
be difficult to block-off air leaks, and harder to test. Crankcase
pressure testing kits are available from Motion Pro.
If I suspect that an engine has an air leak in the crankcases, I do
a visual test. Start by power washing the engine clean. Then remove
the magneto cover. Spray the magneto clean with an aerosol can of
Make sure to use a non-chlorinated type of cleaner (green colored
can). Now spray baby powder to all the suspect areas of the engine.
Spray the powder on the crankcase around the magneto, at the
crankcase seam line, the cylinder base, and the reed valve. Run the
engine for a while, the white baby powder will highlight any fluid
or air leaks on the engine.
The baby powder test is much better than the alternative test of
blowing raw propane gas at different areas of a running engine and
listening for a change in the idle rpm. That is dangerous because it
involves flammable gas and a hot engine with random electrical
A thorough top-end rebuild requires removing the reed valve,
cylinder head, and cylinder. You should tear down your top end
periodically and inspect the reed valve, cylinder head, cylinder,
piston, and so on. Use the following chart to determine when you
should tear down your bike:
Displacement: 80cc 125cc 250cc 500cc
Tear down after: 5 hours 10 hours 20 hours 40 hours
Note that air-cooled bikes should be inspected more frequently.
Also, you may want to inspect more often if you are riding in fine
sand or lots of mud. When you tear down the engine, inspect each
system and look for the following trouble signs.
Check the reed petals for open gaps between the sealing surfaces. In
time, the reed petals lose their spring tension, and the back-flow
can cause a flat-spot in the throttle response. Stock nylon reeds
tend to split at the edges on bikes that are constantly over-revved.
Expert riders find that carbon fiber reeds last much longer.
Check the head at the edge of the chamber for erosion marks-a sign
that the head gasket was leaking. If the head or top edge of the
cylinder is eroded, it must be turned on a lathe to be resurfaced.
All cylinder bases use aligning (dowel. pins around two of the
cylinder base studs. These pins are made of steel, and after heavy
power washing, they get corroded. That makes it difficult to remove
the cylinder from the crankcases. Never use a pry bar! That will
damage the cylinder. Instead use a plastic mallet to hit upward on
the sides of the cylinder at a 45-degree angle. Alternate from left
to right sides so the cylinder lifts up evenly. After you remove the
cylinder, stuff a shop towel into the open crankcases to prevent
debris from entering the engine.
The Different Types of Steel-Lined and Plated Cylinders
There are two types of cylinder bores used on dirt bikes, steel or
cast iron sleeves or ones with plating on the aluminum. Most dirt
bikes made after 1989 have plated cylinders. You can check a
cylinder with a magnet. If it sticks to the bore then it is a
sleeve. If it doesn't stick then it is plated. There are three types
of plated cylinders, Kawasaki Electrofusion, hard-chrome, and nickel
There are several variations of the nickel silicon carbide process
but the most common trade name is Nikasil. The nickel-based
processes have many advantages over hard-chrome, Electrofusion, and
sleeving. Nickel attracts oil and is an excellent carrier material
for silicon carbide particles, a wear resistant material that
carries the load of the piston. This material is electro-plated
right on to the aluminum cylinder for the optimum thermal
Nickel can be honed with diamond stones which leave distinctive
peaks and valley scratches in the cylinder wall which retain oil and
provide a certain bearing ratio between the running surfaces of the
bore. It's possible to rebuild a plated cylinder by fitting it with
However you can expect to pay more for bore maintenance over the
life of the bike, and lose thermal efficiency and horsepower. Plated
cylinders are harder and last longer than sleeved cylinders.
Kawasaki cylinders with the original Electrofusion coating or
hard-chromed cylinders can be repaired with nickel plating or
Steel or cast iron sleeves cannot be nickel plated unless they are
separated from the aluminum cylinder. The reason is that the
pretreatment for the plating would disintegrate the aluminum. There
are four companies that replate cylinders in the USA. The average
price to replate a cylinder is about $200.
Some unfortunate guys do more damage replacing the piston than the
actual wear on the piston! Remove the circlips with a small
needle-nose pliers and throw them away. It is a common mistake to
reuse circlips, but the cheap spring-steel wire clips will fatigue
and break if you install them for a second time.
After removing the circlips, you have to remove the piston pin.
Never use a hammer and punch to remove the pin. That will damage the
connecting rod and needle bearings. Instead, use one of the
pin-extractor tools available from your local franchised motorcycle
shop. You can also grasp the piston with one hand and use a 3/8-inch
socket extension to push the pin out with your other hand.
Too many people replace their pistons too often. The exact service
interval for your bike depends on how hard the bike was run, for how
many hours, the quality of the lubrication, and the amount of dirt
or other debris in the intake air. Bikes that are run hard with
dirty air filters may wear out pistons in only 6 hours, while bikes
that are ridden easy with clean filters and adequate fuel octane may
last 60 hours.
Measuring the Piston
The best thing to do is measure the piston with a caliper. Digital
calipers cost about $100 at industrial tool companies such as Enco
or Harbor Freight. A digital caliper is easy to use and gives
accurate measurements on the piston diameter and cylinder bore.
Measure the widths of the piston (front to back) just above the
intake cutaway because this is the widest point of the piston.
Check the maximum wear specs in your service manual.
Check the piston for detonation marks in the crown, cracks in the
skirt, or seizure marks. Look at the underside of the piston crown
for a large black spot. The spot is burnt oil deposits that adhered
to the piston because the piston crown temperature was too hot. This
is an indication that the carb’s main jet needs to be richer.
Letter Designations on Cylinders and Pistons
The Japanese manufacturers use a letter designation system for
plated cylinders. They intend for you to order replacement pistons
based on the letter designation printed or stamped on the cylinder.
This is the reason why they need this type of system. In mass
production you can't guaranty that all parts will be exactly the
same size. The size variance is based on an acceptable level of
Tool bits become dull, temperatures of machine tools change through
production runs, and machine operators have inconsistent
performance. The Japanese manufacturers have between two to four
different sized pistons and cylinders. Normally labeled A, B, C, and
If they only had one size, the piston to cylinder wall clearance
would vary between .001 to .006 inches. In the standard Japanese
alpha labeling system, A denotes the smallest bore or piston size
and every letter after that is slightly larger, usually in
increments of .0015 inches. The danger is that if you try to put a D
piston in an A cylinder the piston to cylinder wall clearance will
be so tight that a seizure might occur.
Pro-X Oversize Piston Kits
Pro-X is a marketing company that sells the surplus pistons from the
Japanese company ART, which makes all the cast pistons for the
Japanese motorcycle manufacturers. These pistons are the same
quality as the OEM pistons, and they are available in sizes larger
than the alpha pistons available from franchised dealers. Also the
Pro-X pistons are usually priced lower than the OEM pistons. If the
cylinder bore is slightly worn (up to .005 inches) with only a small
area of bare aluminum exposed, you can install a Pro-X oversize
The Pro-X pistons are graded oversize in smaller increments than
Wiseco pistons, but a wider range than the OEM pistons. For example,
Wiseco sizes are .010 inches and Pro-X is .001 inches increments.
Before attempting to order a Pro-X piston, you must measure the
cylinders bore at the smallest point and allow .002 inches clearance
between the piston and cylinder.
Measuring the Ring Gap
The best way to know if the rings are worn is to measure the ring
end gap. Put the ring in the cylinder and use the piston to push it
down about 1/2 inch from the top evenly spaced. Now use a feeler
gauge to measure the width of the ring gap. Normally, the maximum
gap is 0.018-0.025 inch.
Exhaust Valve Cleaning
Does your cylinder have burnt-on mud on the outside, heavy brown oil
glazing on the cylinder bore, or gooey oil on the exhaust valves? If
so, here is a tip for cleaning those parts without flammable
cleaners. Go to the grocery store and get a can of aerosol oven
cleaner. This stuff is great for cleaning the carbon from the
exhaust valves without completely disassembling them. CAUTION: Oven
cleaner attacks aluminum, so don’t leave it on the cylinder for more
than 20 minutes. Oven cleaner can be used on both steel and plated
The oven cleaner will help loosen the oil glazing on the cylinder
walls. Then, you can use a Scotch-Brite pad to hone the cylinder
walls in a crisscross pattern. Wear rubber gloves when you use oven
cleaner and flush the cylinder afterwards with soapy water. This
will neutralize the acid in the oven cleaner and break the molecular
bond of the oil, so the debris can be rinsed away. Sleeved
(especially Kawasaki cylinder bores) are vulnerable to corrosion
after cleaning. Spray some penetrating oil on the cylinder bore to
prevent it from rusting.
Caution: Certain types of cylinders corrode quickly
after the cleaning process, so spray the bore area with
penetrating oil to displace the water.
Honing the Cylinder Bore
Many people have emailed me with questions regarding honing cylinder
bores. If you want to buy a hone to deglaze bores or polish off
small scratches, then a ball-hone is the best choice. Ball hones are
manufactured by Brush Research in Los Angeles, under the brand name
These hones are available under different labels and they are most
easily available from auto parts stores. Buy a size that is 10%
smaller than the actual bore size. These hones are available in
several different materials and grits but the profile that bests
suits both steel and plated cylinders is aluminum oxide 240 grit.
A ball hone cannot remove material from the cylinder bore,
especially on the hard nickel plated bores. However a ball hone can
polish down the peaks of the original hone scratches and increase
the bearing ratio.
In other words the piston will be touching a greater percentage of
the bore. Sometimes that makes the piston wear quicker but if you
have to ball hone the bore to remove scratches, it’s a compromise.
The one type of hone that you should never use on a two-stroke
cylinder is a spring-loaded finger hone. The sharp edges of the
stone will snag the port edges and most likely damage the hone and
Top End Assembly
1. Install one of the circlips in the piston with the
opening facing away in the 6 or 12 o'clock position.
2. Grease the cylinder-base alignment pins.
3. Set the exhaust valves in the closed position.
4. On cylinders with reed valves, leave the intake port open
because you will need to reach in through the port to push the
piston-ring ends back in place.
5. The best way to slip the piston into the bottom of the cylinder
is to rotate the rings toward one side of the locating pins and
squeeze the rings with your middle finger and thumb. That will
leave your other hand free to position the cylinder.
6. There are two methods used to assemble to top end. The first
method is to attach the piston to the connecting rod and lower the
cylinder on to the piston assembly. The second method is to
install the piston assembly into the cylinder and lower the
cylinder and piston on to the connecting rod. The second method is
easier but involves pinning the piston and installing one circlip
with a minimum amount of free space.
7. Take care to align the exhaust valve control mechanism as the
cylinder is bolted to the crankcases.
The oven cleaner you used to clean the cylinders will help loosen
the old gasket material so you can remove it. Carefully scrape the
gasket off with a gasket scraper. Never use a flat screwdriver to
remove the old gaskets because the aluminum surfaces of the head,
cylinder, and crankcases are easily gouged. If these surfaces are
gouged on your engine, they should be draw-filed flat to prevent air
or coolant leaks.
Never reuse paper gaskets; always replace them with new gaskets, and
spray sealer on the paper gaskets, so they will seal better and will
be easier to remove the next time. The new-style steel gaskets can
be cleaned and reused a few times, but you’ll need to spray the
gasket with a sealer such as Permatex Spray-A-Gasket or copper-coat.
Keep a Logbook
Keep a logbook that tracks the number of riding days and the
periodic maintenance. From reviewing the log, you will learn how
often you need to service the top end if you record the measurements
of the ring gap and the piston diameter. A logbook also gives you
greater leverage when you try to sell your used bike for a premium
Big Bore Kits
One of the best ways to increase horsepower is to increase
displacement by overboring the cylinder. This can be ideal for play
or Vet Class riders, where the increased displacement won’t be
illegal for your race class. When done right, a big bore kit can
give you more power everywhere rather than an increase in only the
top or the bottom of the powerband. Such increases are typically
more usable and give you more power where you need it.
Piston manufacturers such as Wiseco make oversize piston kits for
popular model bikes. These kits boost the displacement of the
cylinder to the limit of a racing class or to a larger displacement
class, for example: 80cc to 100cc, 125cc to 145cc, 250cc to 265cc or
300cc, and 495cc to 550cc.
The AMA has a limit of overboring any cylinder used in amateur
modified classes. The limit is 2 millimeters. Wiseco makes a line of
Pro-Lite pistons for this purpose. Normally no head modifications
are needed, but cylinders with exhaust valves that operate close to
the cylinder bore will need to be trimmed for clearance. Cylinders
that use steel head gaskets will require oversize gaskets. Cometic
makes 2 millimeter oversize and big bore gasket kits. The process of
overboring and electro-plating a cylinder can be a cost effective
way to save a cylinder that suffered a top end failure and scored
the cylinder wall.
Riders competing in the AMA veteran class can ride a bike with any
displacement. Riders competing in hare scrambles and enduro can race
the 200cc class with a 125 converted to any displacement. AMA
motocross and enduro racers can make the 250cc bikes legal for open
class by increasing the displacement a minimum of 15 percent (to
Wiseco makes 74-millimeter piston kits to convert the popular 250s
to 300cc. Be careful if you decide to go with a big bore kit,
though. If the overbore is not performed properly, though, it can
result in the wrong kind of power or, at worst, a ruined cylinder.
When you change the displacement of the cylinder, there are so many
factors to consider, such as port time-area, compression ratio,
exhaust valves, carb jetting, silencer, and ignition timing. Here is
an explanation of what you need to do when planning to overbore a
Also, you should at least consult with an expert before tackling a
big bore kit. To get the most from an overbored engine, you need to
make sure the carburetion, exhaust, porting, and timing are all
adjusted to suit the larger bore.
The term port-time area refers to the size and flow range of the
intake and exhaust ports, relative to rpm. The ports enter the
cylinder bore at angles. When the cylinder is over-bored the
transfer ports become lower and wider. The same thing happens to the
exhaust port. This effectively retards the port timing and reduces
the total degrees of duration. When the displacement of the engine
increases, so does the demand for more port-time-area.
If you just overbored and plated a cylinder, it would have much more
low-end power than stock but the top-end power would suffer.
Normally tuners have to adjust the ports to suit the demands of the
larger engine displacement. The proper dimensions for the ports can
be calculated using a computer program from Two-Stroke Racing (TSR)
www.tsrsoftware.com The program "PORTTIME" enables tuners with
limited math skills to run strings of formulas for determining the
optimum dimensions of the ports. Generally speaking, if the ports in
the overbored cylinder were raised to the same heights as the stock
cylinder, that would make the port timing sufficient to run with
stock or aftermarket exhaust systems.
After overboring the cylinder, the head’s dimensions must be changed
to suit the larger piston. First, the head’s bore must be enlarged
to the finished bore size. Then, the squish-band deck height must be
set to the proper installed squish clearance. The larger bore size
will increase the squish turbulence, so the head’s squish band may
have to be narrowed. The volume of the head must be increased to
suit the change in cylinder displacement. Otherwise, the engine will
run flat at high rpm or ping in the midrange from detonation.
When the bore size is increased, the exhaust valve-to-piston
clearance must be checked and adjusted. This pertains to the types
of exhaust valves that operate within close proximity of the piston.
If the exhaust valves aren’t modified, the piston could strike the
valves and cause serious engine damage. The normal clearance between
the exhaust valves and the piston should be at least .030 inches or
The larger the ratio between the piston’s diameter and the carb’s
size, the higher the intake velocity. Overbored cylinders produce
higher intake velocity which draws more fuel through the carb. Of
course a larger engine will need more fuel. Normally when you
overbore an engine 15-20%, the slow jet will need to be richened and
the main jet will need to be leaned. Start with the stock jetting
and make adjustments after you ride the bike.
The ignition timing has a minimal affect on the poweband. Retarding
the timing has the affect of reducing the hit of the powerband in
the midrange and extending the top end over rev. "Overrev" is a
slang term that describes the useable length of the powerband at
The scientific reason for the shift of the powerband to extremely
high rpm, is because the temperature in the pipe increases with the
retarded timing, and that enables the pipe’s tuned length to be more
synchronous with the piston speed and port timing of the cylinder.
Advancing the timing has the affect of increasing the midrange hit
of the powerband, but makes the power flatten out at high rpm. The
reason is that the relatively long spark lead time enables for a
greater pressure rise in the cylinder before the piston reaches TDC.
This produces more torque in the midrange but the high pressure
contributes to pumping losses at extremly high rpm.
Pipe and Silencer
Because only the bore size is changed, you won’t need a longer
pipe-only one with a larger center section. FMF’s line of Fatty
pipes work great on engines that have been overbored.
The head gasket will need to have the bore diameter increased to the
dimension of the new piston. If the head gasket overlaps into the
cylinder bore more than one millimeters on each side, it could
contact the piston or be susceptible to pressure blowouts.
Tips for rebuilding a Two-Stroke Top End
1) Before you disassemble your engine, power-wash the engine
and the rest of the vehicle. That will reduce the risk of dirt and
debris falling into the engine. Once you remove the cylinder, stuff
a clean rag down into the crankcases.
2) The cylinder and head use alignment pins to hold them
straight in position from the crankcases on up. The pins make it
difficult to remove the cylinder from the cases and the head from
the cylinder. Sometimes the steel alignment pins corrode into the
aluminum engine components.
Try spraying penetrating-oil down the mounting studs before
attempting to remove the cylinder and head. Never use a flat-blade
screwdriver, chisel, or metal hammer to remove the cylinder. Instead
use this technique; buy a lead-shot plastic mallet, swing it at a
45-degree angle upwards against the sides of the cylinder.
Alternate from left to right, hitting the sides of the cylinder to
separate it from the cases evenly. Clean the steel alignment pins
with steel wool and penetrating-oil. Examine the pins closely. If
they are deformed in shape, they won’t allow the engine parts to
bolt together tightly. This can cause a dangerous air leak or a
coolant leak. The pins are cheap at about $2 each. Replace them if
they’re rusty or deformed.
3) Never re-use old gaskets. Remove them with a razor blade
or gasket scraper. Don’t use a drill-driven steel wool type pad to
remove old gaskets because they can remove aluminum from the
cylinder and head. That will cause a gasket to leak.
4) Always check the ring end gap on a new ring by placing it
in the cylinder between the head gasket surface and the exhaust
port. The gap should be between .012 to .024 inches.
5) Always install the circlips with the opening facing
straight up or down, that way inertia will hold it tight into the
clip groove. Place one clip in the groove before installing the
piston on the connecting rod. Its easier to install a clip with the
piston in your hand rather than on the rod. There also less chance
that you’ll drop the circlip in the crankcases. Always install the
rings on the piston with the markings facing up. Coat the rings with
pre-mix oil so they can slide in the groove when trying to install
the piston in the cylinder.
6) Always install the piston on the connecting rod with the
arrow on the piston crown facing towards the exhaust port.
7) The traditional way to assemble the top end is to install
the piston assembly on the connecting rod, compress the rings, and
slide the cylinder over the piston. That can be difficult with
larger bore cylinders, or if you’re working by yourself. Try this
method instead. Install one circlip in the piston, install the
piston into the cylinder with the pin hole exposed, install the
piston pin through one side of the piston, position the cylinder
over the connecting rod and push the piston pin through until it
bottoms against the circlip, install the other circlip. It only
takes two hands to install the top end using this manor and there is
less chance that you’ll damage the rings by twisting the cylinder
8) On cylinders with reed valves and large oval intake ports,
take care when installing the piston assembly in the cylinder
because the rings are likely to squeeze out of the ring grooves. Use
a flat-blade screwdriver to gently push the rings back in the
grooves so the piston assembly can pass by the intake port.
9) For steel head gaskets, place the round side of the "bump"
facing up. Don’t use liquid gasket sealer; use aerosol spray
adhesive types instead. For hybrid fiber/steel ring head gaskets,
place the wide side of the steel rings facing down.
10) When you initially start the engine after a rebuild,
manipulate the choke to keep the engine rpm relatively low. Once the
engine is warm enough to take it off choke, drive the vehicle around
on flat hard ground. Keep it under 2/3 throttle for the first 30
Two common myths for proper engine break-in are;
1) Set the engine at a fast idle, stationary on a stand.
When the engine is on a stand it doesn’t have any air passing
through the radiator and it is in danger of running too hot.
2) Add extra pre-mix oil to the fuel.
When you add extra oil to the fuel you are effectively leaning the
carb jetting. This can make the engine run hotter and seize.
Top End FAQs
Thin Sleeve Causing Seizures
Question: My 1987 CR125 has chronic piston seizure problems.
The cylinder is bored one millimeters oversize. The lower end was
rebuilt so I know it doesn’t have a crankcase air leak. What could
the problem be?
Answer: The original cylinder for your model bike had a very
thin steel sleeve. Honda only offers one oversize piston. When the
sleeve is overbored too far, the sleeve cannot transfer out heat
into the water jacket efficiently. The heat builds up over the
exhaust port, and the piston melts. You have two repair options: buy
a new cylinder or install a new thicker sleeve in the old cylinder.
Wiseco offers thick sleeves and forged piston kits.
Honda CR250 1988-91 HPP Problems
Question: My 1990 Honda CR250 is making me wacky. I tried to
check the exhaust valve system, and I don’t think it works properly.
I removed the left-side valve cover from the cylinder, revved the
engine and the valves hardly moved. They don’t open fully when the
engine is revved, and they don’t close completely either. What is
the most common cause of this problem and how can I fix it myself?
Answer: The problem is that the HPP mechanism isn’t fully
engaged, and the valves are just moving from the exhaust-gas
pressure. The most common problem with the 1988-91 CR250 HPP
systems, is the improper engagement of the governor control and the
spindle rod that actuates the HPP valves. The following procedure
may cure the problem.
Remove the top right valve cover on the cylinder and the
round-slotted access cover located under the water pump on the right
side engine cover. Insert an 8mm T-handle through the access hole
and onto the detent bolt that locks the governor control to the cam
spindle, and turn the bolt 1/4 turn counterclockwise.
Now, the bolt has disengaged the HPP system. Insert a straight-blade
screwdriver into the slot in the top of the right-side pinion shaft
(from the top right side of cylinder). Turn the pinion shaft
counterclockwise 1/8 turn, and then turn the detent bolt (located
under the right-side engine cover) 1/4 turn clockwise. It is
important to release the spring tension from the pinion shafts in
the cylinder to engage the detent bolt. This procedure also enables
the HPP mechanism to be engaged without any chance of damage
occurring to the fragile cam spindle.
Top-End Big Bore
Question: I have an old cylinder for my 250. The bore was
ruined when the head gasket leaked, and there is severe erosion on
the top edge of the cylinder. I read your article on top-end
rebuilding and had an idea and a related question. I compete in
amateur enduro events and the rules state that the displacement of
bikes competing in the open class must be a minimum of 251cc. My
question is, can I salvage this old junk cylinder by overboring the
cylinder to fit a Wiseco piston kit and have the bore re-plated? If
yes, will my bike be legal for the open class?
Answer: There are a number of companies offering cylinder
repair services and replating. The way to fix the erosion problem is
to heli-arc weld aluminum over the erosion and then re-face and bore
the cylinder. WISECO and L.A. Sleeve make oversize piston kits and
gaskets for most Japanese dirt bikes. The common overbore
displacement sizes for 250s are 265, 285, and 310cc. After the
cylinder is re-plated, the exhaust valves and the cylinder head must
be matched to the larger bore size. This involves special metal
machining and should be trusted to a qualified tuner or machinist.
This type of mod will enable you to race your 250 in the open class.
Kawasaki Air/Oil Leaks
Question: My son and I are just getting started in
dirt-biking. Over the winter I bought him a 1989 KX80 as a basket
case. We are learning about dirt bike repairs by rebuilding this
bike. It’s a lot like model building, only the parts are old and
greasy! We inspected the crankcases and noticed that there was some
oil leaking from the three oval-shaped plugs that are spaced an
equal distance around the main bearings. How can we repair this
problem without buying new crankcases?
Answer: Every Kawasaki dealer’s service department has a Team
Green book with tips on how to repair common problems. Ask your
dealer’s service manager for a copy of the Team Green bulletin. It
has photos and drawings of how to apply the epoxy over the crankcase
Top-End Seized After Rebuild
Question: I trail ride a 1989 YZ250. Last winter, I rebuilt
the top end after reading your article in Dirt Rider. The bore was
so worn that I had to skip to a one millimeter-oversize piston kit,
just so the bore job would clean up a severely worn spot below the
intake port. After I rebuilt the top end, I cycled the engine by
letting it idle for three 15-minute sessions with adequate cool-down
periods in between. When I first rode the bike, I heard some
detonation noises but didn’t think it was a serious problem, until
it seized. What could be wrong?
Answer: Your problem is simple. When a cylinder is overbored,
the displacement is increased and that boosts the compression ratio.
Whenever a cylinder is overbored more than 0.010 inches or 0.25mms
the cylinder-head diameter must be enlarged to the new bore size.
Otherwise, the piston could contact the head or the edge of the head
surface that extends into the bore could cause a hot-spot and
pre-ignition. Also, the cylinder head’s squish band must be narrowed
by enlarging the combustion-chamber bowl. This also serves to
increase the head’s volume, thereby lowering the compression ratio.
This work must be performed on a lathe by a qualified tuner or
machinist. Average cost of this service is $50
Base Gasket Seeping
Question: I recently rebuilt the top end on my 1991 CR250. I
was being as careful as I could be while taking the cylinder off,
but the dowels were fused in pretty good and I had to pry it.
Needless to say, I gouged the case a bit. I smoothed it out with
sandpaper and reassembled the engine. The bike runs great, but a
little oil seeps out of the cylinder-to-case mating surface. I
assume this is transmission oil? Would it be OK to use something
like a thin layer of Permatex Blue or Yamabond here? Would this make
it even more difficult to remove the cylinder in the future? Should
I just let it alone? The best price I could find on a new left side
case was $215 and I’m sure it would be a lot of work and a lot of
replacing gaskets along the way. Am I out of luck?
Answer: Air leaks can be very dangerous because the engine
could rev independent of the throttle. An inexpensive way to fix
your bike’s problem is to draw-file the cylinder base and the
crankcases. Then apply a thin coating of Yamabond or any other brand
of non-drying sealer to both sides of the base gasket. The best
technique for removing cylinders is to tap up on each side of the
cylinder with a lead-shot plastic mallet. Remember to put a dab of
grease on the cylinder-base dowel pins.
Frequency of Top-End Rebuilding?
Question: I have a 1990 RT180, and I don’t think the rings or
piston have been replaced. I don’t know if the top end has ever been
rebuilt because I bought the bike used. How long do piston and rings
usually last on a two-stroke engine like mine? How often should the
piston and rings be replaced, and should I replace them now?
Answer: Replace the piston and rings before they wear out.
The time scale varies between models, usage, and preventive
maintenance. The only way to determine the condition of your bike’s
top end is to disassemble the top end and measure the piston
diameter and the ring end gap. Compare the measurement to the
maximum wear specs published in the service manual.
Two-Stroke Exhaust Valves
Three words sum up exhaust valve maintenance: spoogey, gooey, and
grungy. If two-stroke exhaust valves didn’t have such a dramatic
effect on the engine’s powerband, I’m sure mechanics would remove
them and beat them bits with a hammer in frustration because there
is little information given by the manufacturers on how to diagnose
and repair the exhaust valve systems on well-used dirt bikes. This
section is a guide to the characteristic mechanical problems that
occur to the exhaust valve systems of dirt bikes. Plus we’ll give
you some tips on how to re-time exhaust valve systems.
How Exhaust Valves Work
An exhaust valve system is designed to increase the engine’s low-end
and midrange power. There are three different designs of exhaust
valve systems. The first-generation design uses a variable-volume
chamber mounted to the head pipe to change the tuned length of the
head pipe. A butterfly valve is used to separate the surge chamber
and the head pipe. At low rpm, the valve is open to allow the
pressure waves in the pipe to travel into the surge chamber and
effectively lengthen the pipe and reduce the pressure wave’s
magnitude when it returns to the exhaust port. These systems were
primitive and not very effective on 125cc dirt bikes. Honda and
Suzuki used this type of exhaust valve system in the mid to late
The second-generation design features valves that control the
effective stroke and the time-area of the exhaust port. These valves
are fitted to the sub-exhaust ports and the main exhaust port. The
main exhaust-port valves operate within close proximity to the
piston to control the effective stroke of the engine. The effective
stroke is defined as the distance from TDC to when the exhaust port
opens. At low rpm, the engine needs a long effective stroke, which
results in a high compression ratio. At high rpm, the engine needs a
shorter effective stroke, longer exhaust duration, greater
time-area, and a lower compression ratio. Yamaha used this system
starting in 1982 on the YZ250. Honda’s HPP system is similar and was
used on the 1986-91 CR250 and 1990 to current-model CR125.
The third generation of exhaust valve systems attempts to change the
exhaust-port velocity, effective stroke, exhaust-gas temperature,
and the pressure of the compression wave. Yamaha and Suzuki started
using these systems on their 125s in 1995. Both companies employed a
venting system to the outside atmosphere. This is very complex
because they are attempting to affect the temperature and pressure
of the returning compression wave to synchronize it with the piston
speed. The exhaust-gas velocity and the effective stroke are
controlled by two oval wedge valves that enter the exhaust port at a
45-degree angle. The wedge valves partially block the exhaust port,
thereby boosting the gas velocity. Kawasaki’s KIPS system uses wedge
valves in the main exhaust port to control the effective stroke,
drum valves in the sub-exhaust ports to control the time-area, and a
surge chamber to absorb the excess compression-wave pressure at low
The exhaust valves are opened and closed by a centrifugal governor
mechanism. The governor is mounted under the right side cover and is
gear-driven by the crankshaft. As the engine rpm increases, the
governor spins, thereby increasing the angular momentum of the four
steel balls encased in the governor. The steel balls fit into an
angled ramp-and-cup arrangement. A spring is used to provide tension
on the steel balls. When the momentum of the steel balls overcomes
the spring’s tension, and the balls force their way up the angled
ramp. A spool attached to the ramp, enabling it to change its linear
position with changes in rpm, and the spool is attached to a linkage
system that operates the exhaust valves in the cylinder. Factory
race teams have different combinations of springs, ramps, and balls
to tune the exhaust valve operation and enhance the powerband.
Exhaust Valve Tips and Tuning
Although exhaust valves use the same essential principles, the
implementation is different with each manufacturer. Also, each type
has its own flaws and fixes. The list below gives you tips on how to
install and service the most common exhaust valves, as well as some
Honda’s HPP system started as a butterfly operated canister mounted
between the cylinder and pipe. It served to control the volume and
length of the exhaust pipe. It had little effect on the power and
most aftermarket pipes eliminated the canister. The butterfly was
prone to carbon seizure and required frequent maintenance. The next
generation HPP was used on the 1986-91 CR250. This system featured
two sliding valves that operated within close proximity of the
piston and effectively varied the exhaust port time-area in
accordance with rpm. The square valves moved horizontally through a
valve guide. The system was plagued with a mixture of design
problems and misinformation on how to service and re-time this
complicated exhaust valve arrangement. This section lists some
common problems and some tips for timing the system, installing the
cylinder, and engaging the HPP mechanism.
Common HPP Problems
Two main problems plague the HPP system: carbon fouling and
rack-and-cam-spindle damage. The square shape of the valves
contributes to the accumulation of carbon on one corner of the valve
guide (stationary part), in the corner of the guide that is directly
in the exhaust gas stream and this causes the valve to become carbon
seized. Chamfering the corresponding edge (one-millimeter) of the
valve will eliminate this problem. The rack and cam spindles are
easily damaged when the cylinder is installed incorrectly, or the
HPP mechanism is engaged incorrectly. See the photos for examples of
damaged rack and cam spindle parts.
HPP Timing Procedure
Use the following procedure to time the HPP system:
1. Install the HPP valves and levers and tighten the pivot nuts.
Place the washer on the stud first, then the lever (marked left and
right), and then the flanged center bushing with the flange side
2. Turn the cylinder upside down. To position the rack correctly,
slide it to the left until it stops; then move it right 2mm. Rotate
the rack so the square notch faces you. Now the rack is in the
correct position so you can install the pinion shafts. Carefully
turn the cylinder right side up without changing the position of the
3. Close the valves and install the left pinion shaft with the
screwdriver slot facing the one o’clock position. Install the right
pinion shaft with the screwdriver slot facing the eleven o’clock
position (see photo for correct positions). A simple way to
determine if the pinions are mis-timed to the rack is to look at the
screwdriver slots. The wrong position is with both slots facing
Installing the Cylinder and Engaging the HPP Drive
After timing the HPP mechanism, the cylinder is ready to be
installed on the crankcases. Here are some tips for installing the
cylinder and engaging the HPP drive mechanism:
1. Make sure the reed valve is removed from the cylinder. CR250s
have such large intake ports that the rings tend to slip out of the
ring grooves during installation of the cylinder. This takes the
spring pressure off the cam spindle. Now turn the engagement bolt
1/4 turn clockwise. You should feel it positively lock into a groove
and stop. Remember that the HPP engagement bolt is a spring-loaded
detent not a threaded bolt. Slide the cylinder down onto the piston
and rings, use a screwdriver to push the rings back in the grooves
until the rings clear the intake port.
2. The HPP mechanism should be engaged while the cylinder is being
installed, just to keep the cam spindle in position. The cylinder
will stop about 3mm from the crankcases because the cam spindle and
the rack are misaligned. Now disengage the HPP mechanism by turning
the engage bolt 1/4 turn counter-clockwise. Grasp the right-side
valve lever and wiggle it; the cylinder should then drop evenly onto
3. Bolt the cylinder down tight. The best way to engage the HPP
mechanism is to insert a screwdriver in the right-side pinion shaft
and turn it counterclockwise. Now turn the engagement bolt
clockwise. You should feel the engagement bolt lock positively in
position. If you try to rotate it too far, you will bend the cam
spindle and the system won’t work at all, so don’t be a hammer-head!
The best way to check the HPP system is to remove the left-side
valve cover from the cylinder, start the engine and warm it up, then
rev the engine. The valves should be fully closed at idle and fully
open when the engine is revved.
In 1992 Honda introduced the HPP system currently used on the CR250.
This system features a center valve for the main exhaust port and
two rotating drum valves to control the flow of the sub exhaust
ports. This system also features a return of the old resonator as
used on the mid-eighties model. The resonator improves the throttle
response and mellows the powerband at low rpm. A thin rod links the
valves together and the whole system is mostly self-scraping to
prevent carbon build-up. The inside of the center valve has an
elongated passage where the tie rod travels. This elongated passage
is prone to carbon build-up over time (1-2 years). The carbon limits
the range of movement in the valves. The carbon is easily removed by
using a small diameter rat-tail file. The sides of the center valve
and the drum valves interface, and that area is prone to carbon
build-up too. A wire brush or file is an effective tool in cleaning
the exhaust valves. Here is a simple way to check the operation of
this system. On the left side of the cylinder there is a 17mm cap
bolt that exposes a straight line mark in the left drum valve. There
is a corresponding mark on the cylinder. The "L" mark denotes the
low speed position of the valve and the "H" denotes the high-speed
position. To check the HPP, start the engine. At idle the valve
should align with the "L" mark. Then rev the engine, the valve
should align with the "H" mark. If the angle of the mark on the
valve is slightly off, then the valve probably needs to be
de-carboned. This system is very easy to disassemble and can only
fit together one obvious way so we won't waste space on that
procedure. There is some aftermarket parts to adjust the performance
of this system for different types of dirt biking. Pro-Racing in
England makes a spacer for the right side valve cover. It serves to
add volume and length to the resonator part of the system. This is
especially suited for enduro riding where a smooth transition to the
mid-range is important for better traction. ESR (Eddie Sanders
Racing) in California makes a replacement HPP system that holds the
valves wide-open. The center exhaust valve is thinner which enables
tuners to raise the exhaust port. The ESR system is primarily used
for dirt track or kart applications where low-end power is of no
Whenever the cylinder is installed on the bottom end after top end
rebuilding, the valves need to be put in the closed position.
Otherwise the HPP cam spindle that connects the actuator in the
cases to the cylinder will get damaged when you tighten down the
cylinder. That will also make the valves inoperable. Always check
the HPP valve operation after you assemble the top end by using the
inspection cap on the left side of the cylinder.
The CR125 HPP system was redesigned in 1990. Honda chose to use a
system similar to the 1986-91 CR250, featuring horizontally sliding
valves. This system was plagued with problems over the years. The
valves are prone to carbon seizure because the critical square edges
face the exhaust stream. If the clips that fit on the ends of the
valves vibrate off or if the valve wears too much then the valve can
tilt on an angle and strike the piston. Another common related
problem happen when tuners widen the exhaust port during porting and
neglect to grind the valves at the outer corners for piston
clearance. There again the piston strikes the valves because they
protrude into the bore. In 1998 Honda made a modification to the
valves, they added an L-shaped rib that prevented the valves from
angling in and contacting the piston. The other problem of clearance
between the top of the valve and the guide was eliminated so the new
style valves provide more low-end power. These valve and guide sets
from the 1998-99 models fit the CR125 models back to 1990.
In 2000 Honda redesigned the CR125 engine and adapted the exhaust
valve system used on the RS250 roadracer. Honda also used this
system on several dual sport and street bikes sold in Asia and
Europe. The new system is so simple and effective. It is a
wedge-shaped valve that pivots at one end, similar to the CR250. The
valve is much thicker and can vary the exhaust port's effective
stroke, time-area, and duration over a wider rpm range. It’s a
self-scraping set up so maintenance should be greatly reduced over
Kawasaki’s KIPS exhaust valve system has gone through a steady
refinement of design. Kawasaki uses a different system to suit the
needs of the different model bikes. The earliest KIPS design used
two drum shaped valves to control the flow of the sub exhaust ports.
Opening the ports gave the exhaust port more time-area. The main
exhaust port was relatively small with modest timing and duration. A
rack and pinion set up controlled the drum valves, opening them at
about 6,000 rpm. Kawasaki uses the rack and pinion design in all
their KIPS systems except the 1998 and later KX80-125cc models.
The 1992 KX125 and KDX used the next generation KIPS which featured
a center wedge valve with two side drum valves engaged to a
rack-and-gear actuating system. This system was very complicated
with all its moving parts. The top and bottom racks had to be
synchronized through the left drum valve, which has two drive gears
molded in it. The drum valves are made of aluminum. When the drum
valve becomes carbon seized, the steel teeth on the rack shear off
the aluminum teeth on the drum valve, rendering the drum valve
Check the condition on the gear teeth every time you do a top-end
service, because if one gear fails the whole system runs out of
sync. . On the late model 80-125cc KXs, the KIPS is relatively
simple relying on a wedge valve and flapper. This system is
self-scraping so it requires little maintenance. In the first year
of operation (1998) the KIPS system was plagued with failures like
the pin breaking on the flapper, the valve receding into the
cylinder and contacting the piston, and over-extension of the valve
causing cock and jam. Pro-Circuit made an aftermarket valve cover
with a full stop that prevented over-extension and in 1999 Kawasaki
changed the wedge valve and flapper design for more rigidity and
that solved all the reliability problems.
The drum valves on the 1988- 92 KX250 and 1990-2000 KX500 are also
aluminum but have a hard-anodized coating that resists wear.
However, the drum valves eventually wear at the drive channels for
the center wedge valve, and the sloppy fit between the wedge and
drum valves prevents the center valve from fully opening. That is
why these bikes get noticeably slower as they get older. There is no
preventative cure or aftermarket part.
You just need to replace the drum valves when the drive channels
wear out. The 1993 KX250 was the first year for the KIPS system used
through present day models. The system uses a single wedge and
flapper valve for the main exhaust port and two drum shaped valves
for the sub exhaust ports. The valves are all linked together with
two racks and pinions on the right drum valve and a steel gear on
the upper rack linking the wedge valve.
A left-hand-thread nut retains the gear to the rod that actuates the
wedge valve. Check the nut periodically, if the nut loosens, the
wedge valves become inoperable. The KX250 KIPS also features two
large cavities to allow for dissipation of the compression wave that
travels back up the exhaust pipe at low to mid rpm. It's important
that the two valve covers on the cylinder be sealed with gaskets and
it is normal for large amounts of black sludge to accumulate under
those valve covers.
It takes years for the sludge to accumulate to the point of
adversely effecting performance. The only way to clean out the
sludge is to have the cylinder hot-tank cleaned at an automotive
rebuilding store. The 1993-2000 KX250 wedge valve tends to form
burrs at the outer edges that face the piston. These burrs prevent
the wedge valve from opening fully, and the thin flap that comprises
the exhaust-port roof hangs out into the exhaust-gas stream,
producing a shock wave that closes off the exhaust port.
File the burrs smooth and check the wedge valve through the full
range of movement. The valve pocket in the cylinder gets worn too.
Aftermarket cylinder rebuilders like US Chrome apply a hard coating
to that area to reduce wear or build-up material that has worn down
from the moving wedge valve. Another characteristic problem of the
KX250 KIPS is broken governor levers. The lever that transmits the
movement from the centrifugal governor to the right-side case lever
tends to break in half. This piece is located under the right side
cover. If your KX250 suddenly loses top end power, its probably due
to the actuating lever breakage or the carbon-seizure of the KIPS
1988-92 KX250 and 1990-2000 KX500 KIPS Timing Procedure
The explanation of this procedure, written in the Kawasaki service
manual, is confusing. It requires you to time the upper and lower
racks at the same instant. My method of timing the exhaust valves is
composed of simple steps that enable you to check your work as you
go. The 1988-92 KX250 and KX500 use the drive-channel system to
actuate the center valve. Here is the best way to time the KIPS on
1. Set the cylinder upside down on a bench.
2. Install the center valve but don’t bolt it in!
3. Install the side drum valves and align the drive channels on
the drum valves with the center valve, but don’t bolt it in!
4. Install the side drums valves and align the drive channels on
the drum valves with the engagement pins on the center valve.
5. Lift up the drum valves so the bottoms of the gears are flush
with the cylinder base. Take care not to disengage the center
6. Slide in the rack from either side of the cylinder. Position
the rack by installing the seal pack and pulling the rack out
until it bottoms against the seal pack. This is the full-open
7. Drop the drum valves onto the rack so the valves are in the
full-open position. Don’t pay attention to alignment dots or marks
on the valve or rack just remember that the valves should be open
when the rack is pulled out and closed when the rack is pushed in.
1992-97 KX125 and 1993-2000 KX250 KIPS Timing Procedure
The system used on the KX125 and KX250 uses both wedge and drum
valves with racks. This is the best exhaust valve system for
performance but the most difficult to maintain. Here are some tips
for re-timing this KIPS system.
1. Install the wedge valves in the cylinder and the
actuating rod and lever. Squirt some pre-mix oil on the parts.
2. Pull the wedge valve into the full-open position, place the
gear on the end of the rod, and rotate the gear counterclockwise
until the rack butts against the stop plate. Thread the nut on the
rod and tighten it counterclockwise because it is a
3. Place the drum valves into their respective cavities until the
top of the gears are level with the cylinder base. Now push the
lower rack into place and bolt the seal pack on the rack into the
4. Pull the rack out until it stops and push it in one millimeter;
now it is in the correct position to install the drum valve.
Before you push down the drum valves, make sure the wedge valve
and drum valves are in the full-open position.
5. Push down the drum valve with the two gears first because it
must engage the upper rack and lower rack simultaneously. Take
care and be patient. You may have to wiggle the wedge valve yoke
to get everything to fall into place. Never hammer the drum
valves! Then push down the right drum valve and install the idler
gear. Now install the bushings and check the system. The valves
will bind and stick if you try to move the valves without the
bushings installed, or if the cylinder is facing upside-down. Test
the KIPS in this way, pull the rack outward until it stops, look
through the exhaust port from the pipe side. The valves should be
in the full open position. On cylinders where the base has been
turned down more than .010 inches, the drum valve bushings will
also need to be turned down to prevent the valves from binding
when the cylinder is tightened.
Suzuki first used exhaust valves in 1985, using a drum valve that
uncovered a cavity in the head or cylinder to add volume and length
to the exhaust pipe, strictly at low rpm. In 1987 they employed a
system that featured two large valves that had multiple functions.
This system was used on the 1989-2000 RM80, 1987-2000 RM125, 1987-95
RM250. The wedge shaped valves was positioned at about a 45-degree
angle over the exhaust port.
The ATEV system is designed to regulate the effective stroke,
exhaust-gas velocity through the exhaust port, and on 1995 and later
models it controls the exhaust gas temperature. The ATEV system is
self-cleaning in that the valves are scraped of carbon every time
they move. Some of the early-model RMs suffered from broken exhaust
valves when the stem would detach from the cylindrical wedge. That
problem was cured in 1991 when the radius between the stem and valve
was increased. The two common problems that occur with the ATEV are
caused by the two following errors in assembling the system:
1. Too much preload on the spring. On the left side of the cylinder
is a dial that controls the spring preload for the exhaust valve
system. The preload doesn’t have that great of an affect on the
engine’s powerband, but too much preload will prevent the valves
from opening, which causes a lack of top-end power.
2. Crisscrossed spring. A centering spring on the right side of the
cylinder, located on the rod, actuates the valves. This spring is
commonly installed wrong. The spring tabs should be parallel when
coupled to the lever and rod. If the spring tabs are crisscrossed,
the valve travel will be limited and won’t open fully.
In 1996 Suzuki redesigned the RM250 engine, going back to a design
reminiscent of the 1987 model RM250. For this model Suzuki modified
Honda's HPP design used on the late model CR250. However a problem
plagued this system. Instead of pivoting the center valve, Suzuki
chose to slide it in a passageway of the cylinder. The added
mechanical friction made the system prone to binding in one
position, half-open. This causes the engine to run flat.
Another problem was the shape of the valve. The leading edge that
faced the piston was too square and sharp. Even when the valve was
in the full open position it caused a shock wave that impeded the
outgoing exhaust flow. Grinding the edge smooth reduced the low-end
power but helped improve top end. In 1997 Suzuki redesigned the
center valve, choosing steel as a material and splitting the valve
into two sections, a major and minor valve.
They also added a two-stage spring system. With some simple grinding
to match the valve to the exhaust port when fully open, this set up
was a winner! Suzuki chose to redesign the system in 1998-2000 to
the 1997 design. The thought was that the steel valve damaged the
valve pocket in the cylinder. Although simply extending the nickel
silicon carbide bore material into the valve pocket would've solved
Yamaha was the first motorcycle manufacturer to adapt exhaust valves
to two-stroke motorcycle engines. Yamaha’s simple design of a
cylindrical valve that rotates 1/4 turn to vary the height of the
exhaust port requires little maintenance. This system was used on
the YZ250 from 1982-98, and on the YZ125 from 1983-93.
Occasionally, you have to replace the seals and O-rings to prevent
exhaust oil from drooling out of the side if the cylinder. In 1989,
Yamaha added a stop plate to limit the travel of the power valve,
primarily so mechanics couldn’t install the valve in the wrong
position. The stop plate is located on the left side of the
cylinder. The valve has a small tab that bumps up against the stop
plate to limit the fully open and closed position of the valve.
This design enabled Yamaha to position the valve closer to the
piston to make it more effective at varying the exhaust-port timing.
Unfortunately, the soft-aluminum tab on the valve gets worn,
allowing the valve to rotate farther in the fully closed position.
Eventually, (after about three years’ use) the tab wears enough so
the valve strikes the piston, causing damage to the piston. Yamaha’s
exhaust valve is cheap to replace. I recommend replacing the valve
when the tab wears more than 0.030 inch (0.7mm).
In 1994 Yamaha changed the engine design of the YZ125 and included
the next generation of exhaust valves. This system used two
oval-shaped wedge valves, positioned at a 45-degree angle over the
exhaust port. This system was similar to the one employed by Suzuki.
Yamaha experimented with resonator cavity volume, and vents for
pressure bleed off and temperature control.
Overall this is a very reliable system. Occasionally the pins that
fit through the ends of the valve to interface with the actuator
lever vibrate out causing the valve to strike the piston. Those pins
are a press fit but you can add some Loctite Instant Adhesive to the
pins for added protection. One problem that Yamaha is concerned with
is high rpm valve flutter. They've added springs to the valves to
control the flutter but future innovations could include a positive
seal between the valve and the cylinders' valve pocket.
In 1999 Yamaha redesigned the YZ250 engine and exhaust valve system.
This model features a powervalve that marks a significant design
change, from the company that pioneered the use of exhaust valves on
two-stroke engines. Looking more like a Rube Goldberg device, the
new powervalve has separate valves for the main (center) and
sub-exhaust ports (sides). The whole assembly is controlled by one
actuating rod, but the side valves open after the main exhaust
valve. The side valves are controlled by two wedge-shaped ramps but
you can bet that the factory teams are experimenting with them. The
stop plate of the center valve tends to crack, allowing the valve to
contact the piston. Look for cracks in the plate richt around the
two retaining bolts.