Twin Twin Yamaha

TWIN TWIN YAMAHA

Four-Cylinder Yamahas Can't Be Bought, So An Enterprising Marylander Built His Own.

ALMOST ANYBODY who followed the 1968 European GP motorcycle season remembers the four-cylinder Yamaha 125 and 250 racers which won the world championships with Phil Read and Bill Ivy riding. But not many have seen a four-cylinder Yamaha 500 in street trim.

Tom Small, of Brookeville, Md., isn't connected with the motorcycle industry in any way. but he is an enthusiast and an excellent machinist. Tom’s machine started life as a 250 Yamaha YDS-3, a model noted for its smooth running, quick turn of speed and slightly heavy weight. At first glance, his machine looks stock, until you spot the four standard exhaust pipes and mufflers.

Work on the engine began with extensive remachining of the two center crankcase halves to accommodate the four-throw crankshaft, which is made up entirely of standard YDS-3 components. No welding was performed in conjunction with the machining operation. The fins were trimmed from each side of the center cylinders and heads, and the standard 99-mm centerline-tocenterline spacing was retained.

The two center cylinders, which are 180 deg. out of phase, breathe through the standard aireleaner, and the two outer cylinders, which are also 180 deg. out of phase, each breathe through Filtron tubular air filter elements.

Four high tension spark coils are triggered by four sets of standard YDS-3 points which have been trimmed down slightly, and are mounted on a plate on the generator in the stock position. Each cylinder’s timing is set using a dial gauge to ensure accuracy.

Frame modifications include centering the engine by relocating some of the standard mounting bosses and by installing others. Four standard YDS-3 exhaust pipes and mufflers are attached in their normal positions and look as though the factory had forseen a conversion like this!

Overall engine width is increased by 7.79 in., and the bike’s estimated weight is 420 lb. Handling is quite good and the acceleration is fantastic, to say nothing of the sound ! fol