Hilborn Technical
Support - Mechanical Systems - By Pass Valves
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Adjusting The Secondary By-Pass Valve
The Secondary By-Pass Valve controls high rpm, closed throttle fuel pressure. The body is marked with the letter "S" and an arrow. 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.
Shims and springs can be added to change the fuel pressure. A thicker wire spring will increase spring tension, resulting in increased fuel flow in the part throttle or closed throttle positions. For example, if the valve is set at 15 psi, that will be the highest pressure the engine will undergo at any rpm with closed throttle.
Since fuel flow is controlled mechanically by the metering valve, any open throttle position 25 degrees or more will automatically shut off the Secondary By-Pass Valve. This eliminates the possibility of a lean condition when throttle is open.
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