Retrospective: Kawasaki 350 S2 Mach II Triple: 1972-1973
December 4, 2012
Filed under Retro + Vintage Motorcycle Reviews
Here was the successor to Kawasaki’s kick-butt A7 Avenger 350, a two-stroke twin using rotary valving. When the A7 appeared, the factory claiming some 40 horsepower, the quarter-mile fanatics were going through the lights in under 15 seconds.
Not bad. But Kawasaki engineers understood that two-stroke power comes from porting and being able to cram more fuel into the combustion chambers…three smaller cylinders offered more porting possibilities than a similar-sized twin.
But rotary valves? Sorry. Doable on a twin, but no way could this be done on a triple without making the engine too wide. So the triples reverted to the tried-and-true piston-port design, which is not only simpler to build, but has better lubrication because the gas-oil mixture spends more time down in the crankcase before going up to the top of the pistons.
The cylinders were aluminum, with cast iron liners, and the pistons fired at 120-degree intervals, making for a smooth engine. Bore and stroke was an almost square 53 x 52.3 mm, with maximum power coming on at 8,000 rpm, redline at 8,500. Thirty lb-ft of torque came along at 7,000 rpm, so there wasn’t much grunt at low rpm. Three 24mm Mikuni carburetors sent the gas to meet with the oil from the Superlube system, controlled by both engine rpm and throttle position. The oil reservoir held five pints, the gas tank, 3.7 gallons.
The S2 was a product of superb machining techniques, with horizontally split cases housing a pressed-together crankshaft using multiple main bearings, three connecting rods and three pairs of flywheels, all tight as the proverbial drum. One of the big concerns was always the carburetion, which needed to be spot on, and the crankcase’s three separate chambers were all part of the system. Any air leak could prove disastrous, and sophisticated rubber seals kept the mixture as it should be.
This minor technical overview really relates to all the Kawasaki two-stroke triples, not just the S2 being described here. There sure were a lot of the triples, beginning with the 500cc H1 Mach III that appeared on these shores late in 1968, to the final 1977 KH400—and who knows how long these bikes were sold in other parts of this far-flung world. The 500 H1 was joined by both the 750 H2 Mach IV and the 350 S2 Mach II in 1972, and then the 250 S1 Mach I in 1973, with the S2 morphing into the 400 S3 in 1974.
This Retrospective is about the 350 version, the S2, which had the shortest run of the lot, only two years. With reason, as will be shown. When Kawasaki began developing the triple concept back in late 1966, the engineers were after one thing, and one thing only—power! Back then, 60 horses out of a 500cc engine was astonishing. So maybe the factory exaggerated a bit, but when a Mach III rider could do the quarter-mile in the low 13 seconds, people paid attention.
Three years later came the (alleged) 74-horsepower Mach IV and (alleged) 45-horse S2, and magazines promoted the S2 as being the most civilized of the trio—and by comparison it was. Back in the early ’70s the 350 racing class was a popular one, both on a club and professional level, being a lot cheaper and somewhat less dangerous than the 500s. Also, some European countries had licensing and tax breaks for motorcycles under 350cc. In the U.S. it did have a price advantage, costing just under $900, while the 500 went for $1,200.
Price was of concern, so the S2 had battery-and-coil ignition rather than the more expensive CDI seen on the 500, and a drum brake up front rather than a disc. No need for a big battery, as the S2—and the others—had a kickstarter. Which an anorexic 90-pounder could kick over with ease. The ignition switch was conveniently up by the speedo and tach, with the spring-loaded enrichener for cold starts right by the throttle. This enrichener had to be held in place to avoid forgetting about it and then fouling plugs, and the bike might need a warm-up of a minute or two on a cool day.
A major difference was that the 350 had some pretense at handling. The frame was a standard double cradle of moderate stiffness, using tubular alloy steel. Up front, the telescopic fork had good springing and rebound damping and was perfect for a solo rider…like on a racetrack. At the back, the shocks had excellent rebound damping and were preload-adjustable. Dry weight was a lightish 330 pounds, more than 50 pounds less than the 500. Wheelbase was short 52.4 inches, the 500 being 55 inches. The Mach II was an entirely flickable bike, with the narrow engine (two inches less than the 500) giving it great cornering clearance.
Essentially, this was a bike that wanted to run at high rpm, with a manual steering damper up by the instruments to aid the speedy types. Full tilt was better than chugging around town, as the throttle had a long pull and if the rider twisted the throttle too fast, the engine could bog…which is why extra plugs were under the seat. Get it up into the six- to eight-thousand rpm range and it was downright fun. Styling was cool, with the long tank flowing into the two-people seat that ended with a sporty kick-up at the very end. The hinged seat lifted up to reveal a tail-box big enough to house tools, extra spark plugs…any small whathaveyous. And there was a very respectable taillight, large enough to keep tailgaters at bay. The handlebar was moderately high, as Americans liked them. The exhaust system had one pipe on the left, two on the right—that look was already instantly recognizable due to the 500. “Wow! That’s one of them triples!!”
A couple of drawbacks: As noted above, Kawasaki was claiming 45 horses and a top speed of 112, but no magazine tester could even get it to 100 mph. Also, it was a thirsty wretch, and if the rider kept the throttle full open he was looking for gas in less than 100 miles. The second problem was the twin-leading-shoe drum brake on the front wheel: Two strokes do not have much in the way of compression braking, and during a spirited ride the brake would begin to fade.
Kawasaki took care of the brake problem by installing a disc late in ’72. But the company, feeling it would sell better as an urban machine, enlarged and detuned the engine, creating the 400cc Mach II in 1974. The horsepower was officially degraded to 42—and the miles-per-gallon were greatly improved.
Year/Model: 1972 Kawasaki 350 S2 Mach II; Owner: Steve Davis, Arroyo Grande, California.
(This Retrospective article was published in the December 2012 issue of Rider magazine.)