Used for main mixture (idle pressure) control. It consists of a body, cap, poppet, spring, jet, and seal. It is usually installed in the line that originates at the fuel pump discharge and terminates at the fuel tank. The poppet acts as a check valve to hold pressure for starting and idle.TECH TIP: 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 largerthe 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 the number is visible to you. This side of the hole has been champfered and is the side that fuel enters.Pit Stop USA offers Hilborn Brass Primary By-Pass Valve F510 at low everyday prices. Be sure to check out all of our products from Hilborn Fuel Injection including Hilborn Fuel Injection High Speed / Low Speed Bypass.