Kawasaki Rotary Valve Engines - FAQ

This page is a FAQ or Frequently Asked Questions specifically about models that feature Kawasaki's 2 stoke rotary valve engines. This page will discuss these Kawasaki models (1965-2001) J1, J1D, J1DL, J1M, J1T, J1TL, B1, B1L, C1, C1L, C1DL, C1TR, C2TR, C2SS GA1A, GA2A, G3SSA, G3TR (Bushmaster), G3TRA, MC1, MC1M, G4TR (TrailBoss), G5, KE100, KS125, KE125, KD125, KX125 (1974-76), KE175, KD175, G31MA. F1, F1TR, F2, F2TR, F3 (Bushwacker), F21M, F5 (Big Horn), F6, F7, F8 (Bison), F81M, F9, 81LA.  Also the twin cylinder models A1 (Samurai), A1SS, A7 (Avenger), A7SS from 1967-1971.  Also included is the AGI (agri) bikes for farm use.

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Here is a 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 expand on things the manual does not. Also you can use this page to find out what Kawasaki motorcycle model you have using the identification guide for the respective displacement (CC or size).

Kawasaki Model identification Guide & specifications

Single cylinder (80 cc - 350 cc) 1965-2001

80cc models

90cc MODELS

100cc MODELS

125cc MODELS

175cc MODELS

250cc MODELS

350cc Models

Twin Cylinder (250 cc - 350 cc) 1967-1971

250cc A1 Samurai

KR250 Tandem Twin

350cc A7 Avenger

Kawasaki "AGI" bikes

Other Brands that featured a rotary valve engine in their lineup of models

Yamaha Rotary Valve bikes

Suzuki Rotary valve bikes

What is a rotary valve?

Rotary Valve Engine (Kawasaki marketing material)

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.

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Rotary valve engine with clutch cover and carburetor installed

Typical Rotary Valve disc (steel version)

Rotary valve engine with carburetor cover installed

Rotary valve disc installed, cover removed. Intake port visible.

Pros & Cons As with any engine design there are pros & cons and a rotary valve engine is no different.

Pros: 
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.

Cons:
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 90cc, 100cc, 125cc differences

Footnote:

Certain Kawasaki 90cc, 100cc, and 125cc rotary valve models made before 1977 had a thermosetting resin (early type of plastic) rotary valve.  These were prone to breaking or had durability issues, especially after 40+ years.  In 1977 (and later) Kawasaki must have realized the durability issues and these models would get a steel version that was backwards compatible.  The larger 175cc, 250cc and 350cc bikes all came with steel valves.  Below is an example diagram of a 1977 model year showing the two options.  The later steel versions require a separate splined collar to retrofit.  The plastic/resin versions had the collar cast as one part.
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Kawasaki really took the rotary valve design to its limit in the early days of the motorcycle boom years.

1970 Kawasaki lineup (all but the H1 triple are rotary valve engines)

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