Snowmobilers have enjoyed the convenience of oil injection since Yamaha outfitted its 1968 SL350 with an oil pump and necessary hardware. No longer did some Yamaha riders have to carry extra oil — and a way to measure it — out into the bush. Instead, they could fill the reservoir and hit the trail without having to worry about the oil can busting open and leaking, not to mention leaving riders without a way to lubricate their sled’s engine. Needless to say, the concept caught on.
The Mikuni oil pump that has delivered lube to so many two-stroke snowmobile engines over the years has undergone very little change. The simple, mechanical pump delivers oil to the engine based on its RPM and throttle position.
Mikuni oil pumps contain a gear-driven cylinder that has a hole drilled through its diameter. The cylinder constantly rotates, which aligns the hole with the discharge passages once every 180 degrees. The pump is driven by the engine’s crankshaft, which means the cylinder in the pump rotates at a speed proportional to the crankshaft. The faster the engine turns, the more frequently the hole through the cylinder aligns with the discharge outlet.
Sitting on top of the cylinder is a small piston that rotates with the cylinder and simultaneously pumps up and down. The stroke length of the piston is determined by the position of a small cam that is controlled by a lever. A control cable links the lever to the vehicle’s throttle lever. As the throttle is opened wider, the small piston’s stroke becomes larger, delivering more oil.
Each engine design has an oil pump that is calibrated for it, and the oil pump cannot be indiscriminately interchanged between engines of different displacement or states of tune. Oil pump control cables stretch and moving parts wear. Mechanical oil injection pumps have index marks on the pump body and control arm. These marks must be aligned when the throttle is in its idle position. The cable must be adjusted properly to align these marks.