LINE ART: 1978 KAWASAKI KR250
Kawasaki struck gold with their KR series, using the likes of Kork Ballington to win a number of world titles, while other riders rode them to success too! Check out Mick Ofield’s lovely line art for this issue!
IN DETAIL:
Most countries (the USA excepted) restricted new street riders to machines of 250cc or smaller. Success at the racetrack was a proven sales tool to attract young riders, making this GP class very important to manufacturers. Walter Villa on a Harley-davidson (H-D owned Aermacchi) pistonported twin dominated the 250GP class from 1974 through 1976.
In 1974 Kawasaki engineer Nagato Sato started designing a competitive engine for this class. He chose disc-valves as a means to produce more power than the piston-ported Harleys.
To overcome the excessive width of a parallel-twin disc-valve engine, Sato chose a tandemtwin layout. The carburettors and disc-valves were on the left, the primary drive, water-pump and ignition rotor on the right. Large main bearings, carburettors and fast port timing required large disc-valves. To accommodate these and keep the engine compact the rear cylinder was offset 16mm to the left, allowing the large disc-valves to overlap.
Both the water-cooled cylinders and heads were siamesed. Two separate cranks were geared together. The front cylinder ran forwards, the rear backwards, thrusting the pistons against the exhaust ports for better sealing. The crankcases, and covers were magnesium.
Cycle parts followed Kawasaki H2R style: twin loop steel tube frame, Morris magnesium wheels, twin shocks and telescopic front forks with brake caliper cast into the slider. The engine attached to the frame at three points with rubber bushings.
This advanced, lightweight 250 debuted at the 1976 Daytona 250 race. To overcome the unique headwind/tailwind problems of this track, the KR250S had seven speed transmissions as allowed under AMA rules. A special selector permitting only six-speeds was used for GPS. Both Ron Pierce and Yvon Duhamel retired with ignition and severe vibration problems. The ignition was easily updated but vibration was so bad frames and engine parts failed, so the bikes were withdrawn from European GP competition.
Vibration came from the cranks being timed at 180 degrees; one piston at TDC while the other was at BDC. The resultant rocking could not be balanced out. Sato redesigned the engine so both pistons rose
“EACH KAWASAKI RIDER NEEDED A KNOWLEDGEABLE TUNER/ MECHANIC AND KORK'S BROTHER DOZY WAS SKILLED IN EXTRACTING MORE POWER AND OPTIMISING THE JETTING.”
and fell in unison with rebalanced cranks. This cured the vibration and resulted in more power. The disc-valves were changed from steel to fibre and ran in coated aluminium housings instead of the original magnesium ones.
The 1977 KR250S had the revised engine and a new chassis. Kawasaki designer Kinou Hiramatsu incorporated the Uni-trak rocker arm rear suspension system from the motocross bikes. Unlike Yamaha’s Monoshock, the Unitrak system could be fine-tuned with different rocker arms, plus ride height was easily adjusted using the links.
Mick Grant finished 8th in the world championship while in the USA Greg Hansford beat Gary
Nixon at Laguna Seca for the KR250’S first major US win.
Two major things occurred in 1978: one, the KR350 and two, signing top 250 privateer, South African Kork Ballington.
The KR350 was basically a KR250 with larger bore (64mm) and 36mm carburettors. KR250S and KR350S were never sold over the counter like TZS but were entered through dealer or distributor teams supported by the factory. This was the era of slick tyre development and the tyre wars between Dunlop and Michelin. Ballington’s Kawasaki UK team used Dunlop, whose front slicks were not initially as good as Michelin’s.
Despite being beaten in the first three 250 GPS, Ballington finally won the 250/350 double at Mugello. Each Kawasaki GP rider needed a knowledgeable tuner/mechanic, Ballington’s brother Deryck (Dozy) was skilled in extracting more power and optimising jetting and gearing. The KR350 was especially troublesome, main bearings failing, cranks only lasting 260 miles and carburation tough to set-up. At the end of the year though Ballington had won both championships!
In 1979 Ballington won both championships again! 1980 saw major changes to the KR250, separate cylinders and heads, plus two extra transfer ports were added to the original three.
Due to illness Ballington missed several rounds, allowing
Anton Mang to take the title on his KR250. Lawson won the AMA 250 championship on his Steve Johnson-tuned KR250.
The following year saw Mang winning both championships, Lawson again winning the AMA 250 championship. 1982 was the last year of competitiveness for the KRS with Mang winning the 350 championship.
These great little bikes without any powervalves or other trick devices scored eight world championships in five years. Quite an achievement from one of the smallest Japanese motorcycle manufacturers!