CLASSIC MOTOCROSS IRON: 1970 KAWASAKI G31M 100 CENTURION
BY TOM WHITE
In the late 1960s, the 100cc class was booming with a multitude of brands filling the starting lines. Kawasaki had previously dabbled in the offroad world with less than stellar results, using mostly modified street bikes. That changed in 1970 when Kawasaki released the G31M Centurion, and soon left all of the other players in the dust. If you went to any 100cc TT (Tourist Trophy) or flat track event in 1970 or 1971, you would see G31M riders on the podium. Top-notch suspension, refined handling and a good pedigree? Nope, just power!
The “Baby Green Streak,” as it was often called, put out an astounding 18.5 horsepower at 10,250 rpm from its rotary disc valve engine. This was nearly double what the Hodaka 100s of the time produced. The stinger on the expansion chamber gave the Centurion a distinct sound that is unmistakable to this day. The engine case was slightly different from other Kawasakis, with the G31M having a secondary air intake on the carburetor side cover. It also required the use of bean oil as a fuel additive for lubrication. Even then, lower-end life was short because of the engine’s high rpm. Never pretending to be anything but a track bike, the G31M was trod with smooth Dunlop K70 3.25-18 front and rear tires. It weighed 178 pounds, had a chrome cylinder bore.
Norm Bigelow, a factory Kawasaki employee for 30 years, was one of the riders who modified the original G31M for motocross. “We used a CZ plastic tank and fenders, replaced the highly tuned cylinder with a mildly ported G4TR cylinder and raced it with some success in the 100cc Novice class at Saddleback Park. Soon the Japanese 125 era started, and the 100 class slowly disappeared.”
Retail price in 1970 was $560. Actor Steve McQueen was given six G31Ms by Kawasaki for the filming of the motion picture Le Mans. McQueen kept one for himself. He painted it orange with gold stripes and gave it the name “Ringadingdoo!” The bike sold at an auction in 2007 at the Petersen Museum in Los Angeles for an unheard price of $55,575; $7000 for a good one is more realistic.
The 1970 model had polished aluminum fenders, and the 1971 version came with painted fenders that matched the tank. The gas tank’s lime green and white design originated with California painter Molly. The stock exhaust pipe is important for collectors. For restorers, it is helpful if the engine of any potential purchase runs, because Centurions were time bombs and engine parts are hard to find. A huge bonus for a collector would be finding the hang tag with dyno results (included on every machine from the factory).