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Hilborn END510S Secondary Bypass Valve, Brass
Hilborn END510S Secondary Bypass Valve, Brass

Hilborn END510S Secondary Bypass Valve, Brass

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Hilborn END510S Secondary Bypass Valve, Brass
Hilborn END510S Secondary Bypass Valve, Brass
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Part # 385F510S
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This brass valve is designed to control high rpm, closed throttle fuel pressure. It consists of a body, cap, poppet, spring, a .187” shim, and seal. 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. Since fuel flow is controlled mechanically by the metering valve, any open throttle position 25 degrees or more will automatically shut of the Secondary By-Pass Valve. This eliminates the possibility of a lean condition when throttle is open. Shims and springs can be added to change fuel pressure.

AN Thread Sizing Guide



A .016 wires 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 mixtrue, use a larger jet size. 

Drop the jet into the valve so the number is visible to you. This side of the hole has been champfered and is the side that fuel enters. 

Some parts are not legal for use in California or other states with similar laws/regulations

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  • Do you have carbs sitting on the shelf waiting to be put on? Heath talks about the preparation and parts needed to install a set of Super 9-7 Carbs on his 1949 Flathead 8BA V8 Model A.

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