Technical Support - Mechanical Systems - By Pass Valves
Return to Technical Support

The Hi-Speed Cutoff Valve

The Hi-Speed Cutoff is an optional valve that controls the higher rpm fuel curve of alcohol fuel engines that exceed 6500 rpm.

There is a certain point where the fuel pump output exceeds the required fuel flow of the engine. This typically starts to happen at 6500 rpm. Increasing the size of the main jet to return this additional fuel will leave the fuel curve very lean in the lower rpm. The Hi-Speed Cutoff is designed to remove this additional fuel without affecting the lower rpm fuel curve. Without the Hi-Speed Cutoff, the engine will seem to "lay-over" and stop accelerating as engine rpm's rev past the 6500 rpm range. Gasoline engines will attribute this to a lean condition but with alcohol the condition is reverse, indicating a rich condition. Using a Hi-Speed Cutoff corrects this high rpm rich condition by bypassing unwanted fuel to the tank, restoring correct engine acceleration.

The Hi-Speed Cutoff uses a poppet and spring arrangement, along with a combination of thick and thin shims to set the required pressure. Tailoring the pressure is accomplished with different shim combinations. The greater the tension on the poppet valve, the higher the pressure in the system. When changes to the main jet in the Primary By-Pass Valve are made, a corresponding change should be made to the Hi-Speed Cutoff. For example, if the main jet is richened .005 (smaller orifice), one thin shim should be added to the Hi-Speed Cutoff, while a thin shim should be removed for every .005 you lean out the main jet.

Installation of the Hi-Speed Cutoff Valve:
The body of the Secondary By-Pass is marked with an arrow and the letter "S". It connects directly off the metering valve with the arrow pointing toward the fuel tank. See the Installation Schematic for location of this valve in the fuel system.