Motorcycle Classics

Rotary-Valve Rocket: 1969 Kawasaki 350 Avenger A7SS

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.

Despite the complexities of rotary valves, their strong power and performance benefits married them to the Kawasaki lineup in one form or another until the mid-1970s. It’s worth mentioning that years later, Barry Sheene’s Suzuki RG500 and Kenny Roberts’ Yamaha YZR500 factory Grand Prix bikes used not just one or two, but four rotary valves apiece. Rotary valves weren’t just Weird Alice engineering; they were real solutions used by factory teams when top performance mattered.

Kawasaki used rotary valves in a broad mix of bikes including the 250cc F21M scrambler and the early KX125, in dual-purpose bikes from 90cc all the way up to the 350cc Big Horn, and in the twin-cylinder 250cc Samurai A1 and 350cc Avenger A7 street bikes and their SS-model street scrambler variants. The 3-cylinder Mach III and Mach IV eschewed rotary valves, due to the impracticality of fueling the center cylinder through such a device. And even if that were accomplished, the engine width would have been too extreme.

Street scrambler heritage

Before dedicated offroad bikes such as the Yamaha DT-1 arrived, street-based scramblers were the best game in town for most riders, leaving hardcore types to pursue a few rare and quirky British and European brands such as Greeves and Husqvarna.

Among the extensive list of available Japanese products, the Honda CL72 and CL77, Kawasaki 250 Samurai SS and 350 Avenger SS offered the best style. (Not to be outdone though, Norton edged out the others with its dramatic high-pipe 750 Commando S: The Norton advertising girls may have helped.) Still other choices included the Yamaha Big Bear Scrambler, Bridgestone GTO and Suzuki X6 Hustler, and Honda later added more CL models.

By today’s offroad standards, street scramblers were mostly showboats, suitable for street and modest forays onto dirt roads and trails. Perspective helps here, because at the time the major emphasis in offroad riding wasn’t so much speed and aerobatics, but simply the ability to master the terrain. And the adequate power, wide-ratio gearboxes, ample ground clearance and better tires offered by scramblers were the primary tools to achieve it.

Today, the singular most identifiable feature of big street scramblers like the Avenger SS is their twin high-rise exhausts, all-purpose tires and a tall, cross-braced handlebar. On the Kawasaki SS, the chromed head pipes twirl out of the cylinders like smoke wisps from a snuffed candle, then make a U-turn around the frame downtube before thrusting up then straight back in parallel like a finely finished over/under shotgun, their bright, ventilated heat shields making them look even more purposeful, even dangerous. You couldn’t miss noticing those pipes, then or now.

About this Avenger SS

California restorer Bill Masho found this particular two-owner A7SS in decent and 98 percent complete condition, with less than 8,500 miles on the odometer. It was a runner, but he describes it as only a four or five on a scale of one to 10 in terms of presentation. He acquired it in January 2013 and restoration took 13 months, with various pauses in between stages of focused work.

Through his business, Vintage Moto Factory (vintagemoto, Masho has done some 41 restorations over the last six years and has his process down to a pretty exact science. First and foremost, he looks for restoration candidates that are complete or nearly so, because he insists on using original parts whenever possible — and starting with a complete bike makes this easier.  The process starts with a complete disassembly of the engine, suspension and chassis. From there Masho creates a list of needed parts and services, and then sends out the components requiring outside talent, such as paint work and machining. “It’s a full disassembly and evaluation of the condition of parts first,” he explains. “Then I try to get things sent out to the outside services right away. Primarily this gets the project going, but it also means those parts don’t take up shop space.”

Masho relies on seven separate specialty vendors to complete a machine. These include chrome plating, zinc plating, painting, powder coating, cylinder boring, metal burnishing, and metalwork and fabrication. Masho does all the metal polishing himself, in this case including the stainless steel fenders, and most all of the cast-aluminum parts such as the big brake hubs.

Masho describes his process as, essentially, remanufacturing a bike. On the Avenger, every nut and bolt, bearing, seal and wire was touched in some way to return the machine as closely as possible to original. “I like to pay attention to details, and would like the bike to come out looking like it did on the dealership floor,” he says. “I also try to stay away from over-restoring, for instance in the paint work. It’s hard to make modern paint look authentically ‘60s or ‘70s, but in this case the painter color-matched the tank to an area of undisturbed original paint on the underside of the tank. I also don’t try to sand, polish or paint over every imperfection. On this bike the rear brake lever had been slightly scraped in a low-speed fall at one point. That’s OK with me, as it is a small bit of evidence that the bike is real and has had a life.” On this particular bike, Masho left the exhaust pipes, rims and rear brake pedal original, but rechromed certain fasteners, the handlebars and other parts to original.

Since he restores bikes for clients, he is fastidious about every detail and goes to great lengths to make sure they are correct. The proof is in the viewing, and the Avenger looks just plain right. Everywhere you turn the components are correct, from the NOS grips he located to the proper 3.25 x 18-inch front and 3.50 x 18-inch rear tires, sold by Coker Tire in the old Dunlop K70 pattern. Masho’s attention extends to choosing the exact sheen of the frame paint, the tone of the soft-looking zinc plating found on certain fasteners, spokes and nipples, and the authentic appearance of the inner and outer engine cases, wheel castings, cylinders and heads — all of which require a different restoration process. Taken as a whole, all of these details end up producing a motorcycle that is positively riveting to look at — and which runs to perfection.

Fast and fun

Masho is also a street rider and a part-time motocross racer; he was preparing to drive across the U.S. to Unadilla, New York, to race his Ossa Phantom during the Avenger photo shoot. He knows how to build engines that run right, and the A7SS does, thanks in part to the Kawasaki’s advanced (for the day) electronic CDI ignition, which provided a super-hot spark.

The engine starts first kick with only modest effort required. It settles down to a coarse, throaty idle and revs quickly. The engine vibrates at all times when it’s running, but whether at idle or redline it is never extreme. It’s a type of vibration that is a pleasant companion and not a detriment. Although the engine is solid-mounted, the bar risers are rubber-mounted, helping to civilize the experience. The powerband is energetic, with minimal torque at low rpm and then building as the rotary-valve intake and exhaust-port timing come into sync. In sum, the Avenger feels weak at low revs, decently rideable in the midrange and delivers a hard-rocking thrill at high rpm.

The engine note is fantastic, uniquely characteristic to the rotary-valve twins that Kawasaki and Bridgestone produced. It is raspy and urgent, impatient at low rpm, with a renegade whoop from the intake canister and a banshee wail from the pipes at high rpm. The twin pipes are unobtrusive and do not get hot, thanks to their heavy steel construction and full-length heat shields.

Still on their original linings, the brakes are, quite frankly, poor, which is to say typical for drum brakes of the period. Proving that it lacks feel as well as power, the front lever effort is also heavy.

Having raced bikes of this type in the 1970s on fast tracks such as Riverside and Ontario, I can vouch that even with careful servicing and adjustment, most drum brakes simply weren’t all that good. The Honda CB750 Four, which popularized disc brakes on motorcycles the same year as this Avenger was built, was the writing on the wall for the iconic drums. Even so, Kawasaki continued with drum brakes on its range-leading 3-cylinder 500cc H1 until a disc finally arrived in 1972.

In terms of creature comforts, by the time the Avenger arrived in 1967, Honda was already giving all its competitors fits. Compared to the previous gold standard, British bikes, they barely leaked oil, they were fairly smooth, had bright lighting and good component quality. So machines like the A7 had to compete not so much with the British but with Honda — and this required being on their best behavior to meet the high bar Soichiro’s products had already set. To do so, the A7 has both a sidestand and a centerstand, a comfortable dual saddle with a passenger grab strap, a lighted tachometer and speedometer with trip meter, automatic oil injection and optional turn signals.

However, it does not have the electric starter prevalent on larger Hondas of the day. The magnificent 903cc Z1 of 1973 would change that for Kawasaki, but that is another story.

In the end, the Kawasaki A1 and A7 rotary-valve twins enjoyed a short five-year run at the top of their respective displacement classes. Soon, tightening emissions laws would change the face of motorcycling until virtually no 2-strokes remained, except for a handful of 50cc scooters and mopeds.

Seeing any 2-stroke on the street today is rare, and a still rarer sight is a twin or triple. Somewhere near the top of the 2-Stroke Spotters’ Guide I just this second invented has to be a rotary-valve twin. So the next time you’re lucky enough to catch a glance of an Avenger streaking past your car window, don’t think of it as just another cool classic bike. Think of it as the era’s highest expression of middleweight 2-stroke performance, during a time when manufacturers could build and sell their engineering visions largely unfettered by regulations. And if the bike just happens to have a bristling pair of chromed over/under high-rise pipes on the left side, you’ve just been zapped by a Kawasaki A7SS, the holy grail of 2-stroke street scramblers. MC

  • Published on Aug 15, 2014
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