Technical Support - Mechanical Systems - By Pass Valves
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The Secondary By-Pass Valve

The Secondary By-Pass Valve is used primarily for applications that get off and on the throttle at high rpm, such as sprint cars or applications with very low stall speed convertors. If the throttle is lifted to an idle suddenly at high rpm, the pump will continue delivering large quantities of fuel since the engine rpm is still high. Without the Secondary By-Pass, one can experience stumbling or excessive fueling at part throttle. With the Secondary By-Pass, this unwanted fuel is returned to the tank, and the system maintains the preset pressure.

Much like the Primary By-Pass, the Secondary By-Pass uses a poppet and spring arrangement, along with shims to help set the required pressure. Tailoring the pressure is accomplished with different spring tensions and shim combinations. The greater the tension on the poppet valve, the higher the pressure in the system.

Since fuel flow is controlled mechanically by the metering valve, any open throttle position of 25 degrees or more will automatically shut off the Secondary By-Pass. This eliminates the possibility of a lean condition when the throttle is open.

Installation of the Secondary By-Pass 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.