Rotary-Valve Rocket: 1969 Kawasaki 350 Avenger A7SS

Before offroad bikes arrived, scramblers like this Kawasaki 350 Avenger were the best game in town.

| September/October 2014

For Japanese motorcycle manufacturers, the 1960s were like the early 1900s for U.S. car companies, with bike makers eager to license, adapt or invent new technology to make their products stand out and attract buyers in a rapidly expanding marketplace.

During this growth period, most Japanese companies focused on 2-stroke engines. Simpler and cheaper to build than 4-strokes, they also offered smaller and lighter packaging, and better performance for the displacement. As well, in the 1960s there were no regulations that particularly affected relatively dirty-running 2-strokes, which gained plenty of notoriety due to their attendant mosquito-fogging trails of blue smoke. Building “‘strokers” became the accepted industry-wide means of delivering the power that customers wanted, and all the major players except Honda focused on developing them. Honda may have eventually enjoyed the last word on the matter, but even so, 2-strokes held on until the early 1980s.

Rotary valves: genius or enigma?

One of the most flamboyant of those 1960s 2-stroke engineering companies was Kawasaki. Its range included rotary-valve induction singles and twins such as the lovely 1969 350 Avenger A7SS twin seen here. And they were fast. Until the arrival of the vaunted Kawasaki H1 500cc triple, virtually nothing else on the street — regardless of displacement — could dispatch the Avenger in a quick stoplight fight.

Kawasaki’s rotary-valve engines are traditional 2-strokes with the exception that they use a spinning disc valve, mounted on the crankshaft in between the side-mounted carburetor and the crankcase, to control the intake timing. Looking like a phenolic pizza cutter with a big piece missing, the whirling disc sweeps across an opening in the side of the crankcase, first exposing and then chopping closed the portal at precise intervals. The carburetor or carburetors mount outside of the sealed rotary-valve housing on one or both sides of the engine.

The two primary advantages to rotary valves include better control over intake-port timing and an unobstructed flow of the incoming charge into the crankcase. Renowned 2-stroke engine builder Scott Clough explains: “With a piston-port intake design you’re stuck with the same opening and closing timing, but with a rotary-valve engine the opening and closing of the intake port timing are fully independent,” he says. “In addition, unlike in a reed-valve engine, the port opening is completely unobstructed. As a result, rotary-valve engines can develop a broader power range and produce more peak horsepower than similar piston-port or reed-valve engines.”

But there are also a few drawbacks. Compared to a traditional piston-port or reed-valve engine, rotary-valve engines are heavier, wider and cost more to make. This is particularly true for a twin; the Kawasaki A7 engine measures 18.5 inches wide compared to 16 inches for a Honda CB350 of the same vintage. This difference isn’t enough to bother street riders, but in the dirt or on a racetrack, any additional width is an impediment. To make its rotary-valve enduro bikes work better offroad, Can-Am later positioned the carburetor behind instead of beside the cylinder, moving the air/fuel charge through a longer path before reaching the crankshaft-mounted rotary valve.

11/6/2014 11:44:50 PM

had the road version when it was only about 3 years old-what a rocket, i cut my rotary valves to the same spec as the A7R production racer- theyre only a bakelite type material, i then went down 2 teeth on the rear sprocket making it good for around 115 mph and still able to stick a wheel in the air in first 2 gears no worry- tail light replacement was a regular thing. just so much fun! then the RD Yamahas came along and they handled the corners better.Had a 250 as well,nearly as fast- revd harder.

10/30/2014 8:54:25 AM

Oh, yeah, they were fast. I remember reading in the magazines how the 350 Kawasaki could out-stoplight anything on the street, and it could. At the time, I had a 175cc Bridgestone Dual Twin, another rotary-valve two-stroke twin, with a funky 4 and 5-speed gearbox. One time, I stretched my 120-pound frame lengthwise, putting my feet on the luggage rack, chin on the tank, and my hands on the fork tubes (there was a thumbscrew throttle lock). Traveling several miles at full throttle, I eventually saw an indicated 100mph. (I recall it was a 100mph speedo, but I may be wrong there.) Great, except I couldn't figure out how to get my hand back to the throttle! Whether that speedo was accurate at that speed (and I suspect it wasn't), that little bike would go. And the Avenger was way, way faster. With pretty much equivalent brakes. Oh, boy!

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