Kawasaki Rotary Valve Engines - FAQ
I created this page to be a FAQ or Frequently Asked Questions specifically about models that feature Kawasaki's 2 stoke rotary valve engines. It would seem there was no centralized repository for specifications, technical, tuning, how to and repairs that are unique to a rotary valve engine. While the information contained here is specific to each model discussed, a lot of the information is universal enough to apply to other brands that used this engine design in motorcycles, PWC's and snowmobiles. Early Yamaha motorcycles had rotary valve 2 stroke engines as did Suzuki, however they mostly abandoned the design by the late 60's in favor of piston port or reed valve 2 strokes. Another notable brand was CAN-AM, but they are more outliers as it was Kawasaki that really developed the rotary valve 2 stroke to probably its fullest potential. As always, consult the appropriate manual for your model for even more detailed information. This site is designed to help de-mystify certain common questions and explain on things the manual does not. This page will discuss these Kawasaki models GA1A, GA2A, G3SSA, G3TRA, MC1, MC1M, G4TR, G5, G31MA. F5, F6, F7, F8, F81M, F9, 81L.A
 Typical Rotary valve disc
What is a rotary valve?
A rotary valve as used in a 2 stroke motorcycle engine is simply a round disc with a specific cutout around the perimeter that is keyed to the crankshaft. The purpose is to function as a means of closing off the intake tract at a specific time and duration. Think of it similar to how a traditional intake poppet valve in a 4 stroke engine controls the incoming air/fuel mix into the cylinder. The biggest difference is that the intake on most* rotary valve style engines is on the right side, just above the crankshaft with a straight shot right into the crankcase. *Some notable exceptions being Kawasaki twins A7, A1 (Avenger, Samurai), vintage Can-Am/Rotax engines are on the left side, older 2 stroke See-Doo and other PWC 2 stroke twins can have them on the front transverse to the crankshaft driven by a set of bevel gears. Irregardless the function and principle is the same. 
Rotary valve engine with clutch cover and carburetor installed
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Rotary valve disc installed, cover removed. Intake port visible.
Rotary valve engine with carburetor cover installed
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Pros & Cons As with any engine design there are pros & cons and a rotary valve engine is no different. Below are the main ones
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Pro:
Precise control of the intake timing versus other 2 stroke designs. Straight intake flow directly into the crankcase for efficiency Lower engine center of gravity with side mounted carb. Intake timing can be asymmetric in respect to exhaust port timing. Crankcase pressure and thus scavenging can be improved. |
Con:
More complex design, more sealing surfaces and chances for air leaks. Intake runners are very short and can hinder intake velocity for certain applications. Engines are wider because the carburetor(s) need to be mounted on the side(s) of the engine. Rotary Valve Disc can experience wear and need to be replaced Generally nosier versus piston port & reed valve designs. (subjective) |
Kawasaki Engine Model Overviews
Single cylinder (90 cc - 350 cc)
Use the slider, pick the displacement and see information about the models within that displacement
My Kawasaki MC1M
90 CC
GA1A, GA2A, G3SSA, G1, G1L, G1DL, G1TRL, MC1, MC1M
J1, J1L, J1T 85cc
All the 90cc engines, generally speaking, share a lot of internal and external parts. Later models that superseded earlier models usually have some subtle changes to either improve performance OR durability. It is within these subtleties of specifications where certain models can be harder to find specific parts.
2 stroke Oiling System: Super-Lube (can be disabled for premix use)
Ignition: Contact Breaker Point
Transmission oil: SAE 10W30 or 30
GA1A, GA2A, G3SSA, G1, G1L, G1DL, G1TRL, MC1, MC1M
J1, J1L, J1T 85cc
All the 90cc engines, generally speaking, share a lot of internal and external parts. Later models that superseded earlier models usually have some subtle changes to either improve performance OR durability. It is within these subtleties of specifications where certain models can be harder to find specific parts.
2 stroke Oiling System: Super-Lube (can be disabled for premix use)
Ignition: Contact Breaker Point
Transmission oil: SAE 10W30 or 30
Kawasaki G4TR
100 CC
D1, G3TRA, G4TR (Trail Boss), G5, KE100, KM100, G31M (Centurion)
All the 100cc engines, generally speaking, share a lot of internal and external parts. Later models that superseded earlier models usually have some subtle changes to either improve performance OR durability. Some of these 100cc models also are very closely related to some of the 90cc models. Some of the 100cc models share lower end and engine covers with the 90cc models. It is within these subtleties of specifications where certain models can be harder to find specific parts.
The one unique feature on the G4TR is the HI/Lo range transmission, meaning it has a theoretical 10 speeds to choose from. A tell tale sign is an extra cover bolted to the countershaft cover that house the HI/Lo range selector mechanism.
2 stroke Oiling System: Super-Lube (can be disabled for premix use), G31M was designed without an oil pump, premix only
Ignition: Contact Breaker Point
Transmission oil: SAE 10W30 or 30 (D1, G3TRA), 30 or ATF (G4TR, G5, G31M, KE100, KM100)
It should be noted that the G31M Centurion has very few similarities with these other 100cc bikes, most of the internal parts are unique including the plated cylinder (for reduced friction and heat dissipation), thus giving it the 18.5 hp (claimed) versus all the other models roughly 11 horsepower. One of the standout features is the auxiliary air intake screen cut into the side of the carburetor cover (similar to the F81M 250cc)
D1, G3TRA, G4TR (Trail Boss), G5, KE100, KM100, G31M (Centurion)
All the 100cc engines, generally speaking, share a lot of internal and external parts. Later models that superseded earlier models usually have some subtle changes to either improve performance OR durability. Some of these 100cc models also are very closely related to some of the 90cc models. Some of the 100cc models share lower end and engine covers with the 90cc models. It is within these subtleties of specifications where certain models can be harder to find specific parts.
The one unique feature on the G4TR is the HI/Lo range transmission, meaning it has a theoretical 10 speeds to choose from. A tell tale sign is an extra cover bolted to the countershaft cover that house the HI/Lo range selector mechanism.
2 stroke Oiling System: Super-Lube (can be disabled for premix use), G31M was designed without an oil pump, premix only
Ignition: Contact Breaker Point
Transmission oil: SAE 10W30 or 30 (D1, G3TRA), 30 or ATF (G4TR, G5, G31M, KE100, KM100)
It should be noted that the G31M Centurion has very few similarities with these other 100cc bikes, most of the internal parts are unique including the plated cylinder (for reduced friction and heat dissipation), thus giving it the 18.5 hp (claimed) versus all the other models roughly 11 horsepower. One of the standout features is the auxiliary air intake screen cut into the side of the carburetor cover (similar to the F81M 250cc)
The unique G31M Centurion, shares very few parts with other models
Kawasaki F6
125 CC
B1LA, F6, KS125, KD125, KE125
The F6 is the more popular version of the 125cc bikes. The F6 shares very little if almost nothing with the B1LA. It shares nearly everything with the F7 as it is just an F7 lower end with a smaller piston and cylinder. It would appear that the F7 was developed first and then downsized to fit a model into the 125cc category at the time. The B1 has electric start, a unique feature for a 2 stroke motorcycle. The KS125/KE125 was similar to the F6 but with a bit lower horsepower, thus they don't share too much in the way of the internals.
2 stroke Oiling System: Super-Lube (can be disabled for premix use)
Ignition: Contact Breaker Point
Transmission oil: SAE 10W30 or 30 (All Models)
B1LA, F6, KS125, KD125, KE125
The F6 is the more popular version of the 125cc bikes. The F6 shares very little if almost nothing with the B1LA. It shares nearly everything with the F7 as it is just an F7 lower end with a smaller piston and cylinder. It would appear that the F7 was developed first and then downsized to fit a model into the 125cc category at the time. The B1 has electric start, a unique feature for a 2 stroke motorcycle. The KS125/KE125 was similar to the F6 but with a bit lower horsepower, thus they don't share too much in the way of the internals.
2 stroke Oiling System: Super-Lube (can be disabled for premix use)
Ignition: Contact Breaker Point
Transmission oil: SAE 10W30 or 30 (All Models)
Kawasaki F7 175
175 CC
F2, F2TR, F3 (Bushwacker), F7, KE175
Probably the most recognized and well known of the 175's is the F7, however a few models preceded it paving the way for the F7's (claimed) 21 hp. Unlike some of the other smaller displacements the F7 shares very little with the previous models. It could be considered that the F7 is a highly refined and more developed version of the F3 even thought they share almost nothing. The KE175 is a lower horsepower version of the F7 it shares a few things like the crankshaft and connecting rod assembly, but the pistons are different. Read here about the "drain pump" that can fail causing jetting problems and even piston seizure.
2 stroke Oiling System: Super-Lube (can be disabled for premix use)
Ignition: Contact Breaker Point (F2, F2TR, F3) CDI (F7, KE175 these are not compatible with each other)
Transmission oil: SAE 10W30 or ATF (All Models)
F2, F2TR, F3 (Bushwacker), F7, KE175
Probably the most recognized and well known of the 175's is the F7, however a few models preceded it paving the way for the F7's (claimed) 21 hp. Unlike some of the other smaller displacements the F7 shares very little with the previous models. It could be considered that the F7 is a highly refined and more developed version of the F3 even thought they share almost nothing. The KE175 is a lower horsepower version of the F7 it shares a few things like the crankshaft and connecting rod assembly, but the pistons are different. Read here about the "drain pump" that can fail causing jetting problems and even piston seizure.
2 stroke Oiling System: Super-Lube (can be disabled for premix use)
Ignition: Contact Breaker Point (F2, F2TR, F3) CDI (F7, KE175 these are not compatible with each other)
Transmission oil: SAE 10W30 or ATF (All Models)
My F81M replica
250 CC
F4 (Sidewinder), F21M (Green Streak), F8 (Bison), F81M
The F4 and F21M both share quite a bit of engine and internals, just as the F8 and F81M share a lot of the same parts. However the two groups do not share anything between the two. It could be considered that the F8/F81M engines are more developed and refined versions of the F4/F21M. The F8/F81M share a lot with the larger 350cc F5 engines. Most parts are the same except for the piston and cylinder. Fun fact! The muffler/expansion chamber on the F81M is the same one that was offered in the "Speed Kit" for the F5 (350cc) Big Horn
2 stroke Oiling System: Injecto-Lube (can NOT disable for premix use, lubricates right side crank bearing) Except F81M designed for pre-mix only
Ignition: Contact Breaker Point
Transmission oil: SAE 10W30 or ATF (All Models)
F4 (Sidewinder), F21M (Green Streak), F8 (Bison), F81M
The F4 and F21M both share quite a bit of engine and internals, just as the F8 and F81M share a lot of the same parts. However the two groups do not share anything between the two. It could be considered that the F8/F81M engines are more developed and refined versions of the F4/F21M. The F8/F81M share a lot with the larger 350cc F5 engines. Most parts are the same except for the piston and cylinder. Fun fact! The muffler/expansion chamber on the F81M is the same one that was offered in the "Speed Kit" for the F5 (350cc) Big Horn
2 stroke Oiling System: Injecto-Lube (can NOT disable for premix use, lubricates right side crank bearing) Except F81M designed for pre-mix only
Ignition: Contact Breaker Point
Transmission oil: SAE 10W30 or ATF (All Models)
My Barn Fresh Restoration Kawasaki F5
350 CC (Big Horn)
F5, F9
The Kawasaki Big Horn! Probably one of the most notorious of the rotary valve singles made by Kawasaki. Technically speaking only the F5 (1970) and F5A (1971) are Big Horn's as only those 2 years were they marketed as such. The F5A would extend to have "350E" designation in marketing literature to denote "Enduro" just like the rest of the model lineup in 1971. Also the "Power Pak" or "Speed Kit" was only available for 1970-71, and dropped for the F9. The "Speed Kit" claimed to raise the horsepower from the advertised 33 to 45. Fun fact! The muffler/expansion chamber, rotary valve disc, in the kit is the same one that came stock on the F81M. This is also true for the carburetor cover and air filter cutout on the side. This was similar to Yamaha's GYT Kit for the DT1. The F9 shares a lot with the F5 but there are few differences in engine covers and a few internal parts. However most things interchange and as a whole the F9 could be considered a more developed version of the F5 but mostly in other ways aside from the engine. Read here about the unique design for evacuating fuel/water from the carburetor chamber on the F5/F9.
2 stroke Oiling System: Injecto-Lube (can NOT disable for premix use, lubricates right side crank bearing)
Ignition: CDI
Transmission oil: SAE 10W30 or ATF
F5, F9
The Kawasaki Big Horn! Probably one of the most notorious of the rotary valve singles made by Kawasaki. Technically speaking only the F5 (1970) and F5A (1971) are Big Horn's as only those 2 years were they marketed as such. The F5A would extend to have "350E" designation in marketing literature to denote "Enduro" just like the rest of the model lineup in 1971. Also the "Power Pak" or "Speed Kit" was only available for 1970-71, and dropped for the F9. The "Speed Kit" claimed to raise the horsepower from the advertised 33 to 45. Fun fact! The muffler/expansion chamber, rotary valve disc, in the kit is the same one that came stock on the F81M. This is also true for the carburetor cover and air filter cutout on the side. This was similar to Yamaha's GYT Kit for the DT1. The F9 shares a lot with the F5 but there are few differences in engine covers and a few internal parts. However most things interchange and as a whole the F9 could be considered a more developed version of the F5 but mostly in other ways aside from the engine. Read here about the unique design for evacuating fuel/water from the carburetor chamber on the F5/F9.
2 stroke Oiling System: Injecto-Lube (can NOT disable for premix use, lubricates right side crank bearing)
Ignition: CDI
Transmission oil: SAE 10W30 or ATF
Below are some overall differences between model years for the Big Horn F5 and the F9
Model designation Key:
F5 = 1970
F5A = 1971
F9 = 1972-1975 (A,B,C for respective years 73-75)
The F5 (first year) was produced in relatively limited numbers at aprox. 7,000 units. Compared to Yamaha producing almost 3 times as many RT1's (360cc)
Early engines had a different kick start mechanism and lever as well as a smaller big end rod bearing size. A changed mid year with engine numbers after 07268 (mid 1970 year change) had larger (more reliable) big end rod bearings. Newer crankshafts will fit earlier engines. My early F5 bike has engine number 02300 and a build date of February 1970
F5 Control cables (clutch, rear brake, and tach) were silver. F5A/F9 (1971/1972-75) cables were black
F5 and F5A cylinders and heads are natural aluminum color, The F9 has black painted cylinders and heads
F5 ,F5A uses a 32mm (large body) carburetor, the F9 uses a 30mm (small body) carburetor. This accounts for the lower HP numbers for the F9, but made the bike a bit mellower which made for a better riding bike.
Carburetor cover and control cable boot are unique to the F5. F5A and later (F9) used a different cover and boot that improved sealing. This design changed twice.
F5 had Painted graphics. Later F5A and F9 used vinyl decals much like other manufacturers.
Handlebar is chrome and bend is unique to the F5 and F5A, The F9 used a different bend and came in black
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Speedometer and tachometer are unique to the F5 with chrome trim rings. F5A gauges had rubber hoods to reduce glare, F9 the gauges changed slightly.
F5 chain guard is silver. The F5A is black, the F9 is black and of a different design.
F5, F5A and F9, F9A featured the Hatta forks, F9B and C changed to the more conventional forks similar to the F11 (1973 piston port 250cc) but still featured the adjustable pre-load, but did not have the adjustable axle position.
Foot pegs are rubber, round, spiked unique to the F5,and F5A. The F9 had a more conventional flat style rubber.
Muffler heat shield is chrome on the F5, black on the F5A both mufflers require optional bolt on spark arrestors. Without the spark arrestor the exhaust exits right before the rear shock. The F9 completely redesigned the muffler with a built in spark arrestor and bends around the rear shock exiting after the shock.
Alloy (aluminum) wheels were featured on the F5 and F5A The alloy wheels were soft (but light) and had a groove or lip that held mud when riding in muddy conditions. The F9 had chrome steel wheels that were heavier but more durable. They also had a more conventional flat profile that shed mud better. The F9 used heavier duty spokes as well.
Front brake drum design on the F5, F5A ,F9, F9A is lighter and compared to the F9B/C. It also uses a brake drum strut. The F9B/C front brake drum dimensionally larger and of a different design on the outside but uses the same brake shoes. It also features a brake wear indicator and a different brake arm actuator and the F5 brake drum strut was replaced with cast in slots that fit into tabs cast into the lower end of the forks. Thus lower fork tubes and/or front wheels are not interchangeable between the two groups of models.
Lightweight aluminum fenders on the F5 and F5A, The F9 changed to steel for durability. The physical designs of the fenders are identical though.
Rear tail lights are different on the F5 versus the F9. The F9 taillight is much heavier.
Model designation Key:
F5 = 1970
F5A = 1971
F9 = 1972-1975 (A,B,C for respective years 73-75)
The F5 (first year) was produced in relatively limited numbers at aprox. 7,000 units. Compared to Yamaha producing almost 3 times as many RT1's (360cc)
Early engines had a different kick start mechanism and lever as well as a smaller big end rod bearing size. A changed mid year with engine numbers after 07268 (mid 1970 year change) had larger (more reliable) big end rod bearings. Newer crankshafts will fit earlier engines. My early F5 bike has engine number 02300 and a build date of February 1970
F5 Control cables (clutch, rear brake, and tach) were silver. F5A/F9 (1971/1972-75) cables were black
F5 and F5A cylinders and heads are natural aluminum color, The F9 has black painted cylinders and heads
F5 ,F5A uses a 32mm (large body) carburetor, the F9 uses a 30mm (small body) carburetor. This accounts for the lower HP numbers for the F9, but made the bike a bit mellower which made for a better riding bike.
Carburetor cover and control cable boot are unique to the F5. F5A and later (F9) used a different cover and boot that improved sealing. This design changed twice.
F5 had Painted graphics. Later F5A and F9 used vinyl decals much like other manufacturers.
Handlebar is chrome and bend is unique to the F5 and F5A, The F9 used a different bend and came in black
.
Speedometer and tachometer are unique to the F5 with chrome trim rings. F5A gauges had rubber hoods to reduce glare, F9 the gauges changed slightly.
F5 chain guard is silver. The F5A is black, the F9 is black and of a different design.
F5, F5A and F9, F9A featured the Hatta forks, F9B and C changed to the more conventional forks similar to the F11 (1973 piston port 250cc) but still featured the adjustable pre-load, but did not have the adjustable axle position.
Foot pegs are rubber, round, spiked unique to the F5,and F5A. The F9 had a more conventional flat style rubber.
Muffler heat shield is chrome on the F5, black on the F5A both mufflers require optional bolt on spark arrestors. Without the spark arrestor the exhaust exits right before the rear shock. The F9 completely redesigned the muffler with a built in spark arrestor and bends around the rear shock exiting after the shock.
Alloy (aluminum) wheels were featured on the F5 and F5A The alloy wheels were soft (but light) and had a groove or lip that held mud when riding in muddy conditions. The F9 had chrome steel wheels that were heavier but more durable. They also had a more conventional flat profile that shed mud better. The F9 used heavier duty spokes as well.
Front brake drum design on the F5, F5A ,F9, F9A is lighter and compared to the F9B/C. It also uses a brake drum strut. The F9B/C front brake drum dimensionally larger and of a different design on the outside but uses the same brake shoes. It also features a brake wear indicator and a different brake arm actuator and the F5 brake drum strut was replaced with cast in slots that fit into tabs cast into the lower end of the forks. Thus lower fork tubes and/or front wheels are not interchangeable between the two groups of models.
Lightweight aluminum fenders on the F5 and F5A, The F9 changed to steel for durability. The physical designs of the fenders are identical though.
Rear tail lights are different on the F5 versus the F9. The F9 taillight is much heavier.
Twin Cylinder (250 cc - 350 cc)
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250 CC
A1, A1SS (Samurai) Twin cylinder rotary valve engine. Carburetors are fitted to both sides of the engine. Shares a lot of parts with the bigger 350cc version the A7. 2 stroke Oiling System: Injecto-Lube (can NOT disable for premix use, lubricates crank bearings) Ignition: CDI |
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350 CC
A7, A7SS (Avenger) Twin cylinder rotary valve engine. Carburetors fitted to both sides of the engine. Shares a lot of parts with the smaller 250cc version the A1 2 stroke Oiling System: Injecto-Lube (can NOT disable for premix use, lubricates crank bearings) Ignition: CDI |
