Technical Support - Mechanical Systems - By Pass Valves
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Adjusting The Primary By-Pass Valve

The Primary By-Pass Valve is used for main mixture (idle pressure) control. The valve body is marked with an arrow and a #4. When the valve is installed, the arrow should point toward the fuel tank. It is usually installed in the line that originates at the fuel pump discharge and terminates at the fuel tank. Installing the Primary By-Pass closer to the fuel tank will alleviate weight (caused by fuel in the line) from pressing against the poppet inside the valve. A long by-pass hose with the by-pass valve installed close to the fuel pump will usually cause a richer than normal idle. See the Installation Schematic for location of this valve in the fuel system.

The poppet inside acts as a check valve to hold pressure for starting and idle. Inspect the poppet regularly for cracks and dirt, which can cause sticking.

A .016” wire spring is the most commonly used, however, .020 and .024 are also available. Using a thicker wire spring will give more spring tension, resulting in a richer fuel mixture at idle and low rpm operation.

The jet (pill) controls the amount of fuel the engine will receive at all rpm ranges. The larger the jet size, the more fuel is allowed to return to the tank, therefore leaning out the mixture. To richen the mixture, use a smaller jet size. To lean the mixture, use a larger jet size. Drop the jet into the valve so that the number is visible to you. This side of the hole has been chamfered and is the side that fuel enters.