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

The Primary By-Pass Valve supplied with your Hilborn Injector serves two purposes. First, as a restriction in the system to help ease starting and, secondly, to house the Main Jet.

For Starting:
Remembering that a constant flow system needs a restriction to develop pressure, the spring and poppet valve located in the Primary By-Pass effectively block off the main jet in order to help develop pressure during cranking when pump speed, and therefore pressure, is very low. Springs for the poppet valve are available in 3 different gauges, allowing us to tailor the amount of fuel needed for each application. Most gasoline applications will use a .016 gauge spring (1.5 pounds of pressure), while alcohol applications will use either a .020 or .024 gauge (3 pounds or 5 pounds of pressure). Regular inspection of the poppet valve for cracks and dirt is recommended for proper operation.

Housing the Main Jet:
The Primary By-Pass valve also houses the Main Jet, or pill. The Main Jet controls the amount of fuel the engine will receive at all RPM ranges, along with setting the fuel curve at WOT. The larger the main jet, the more fuel is returned, and the leaner the mixture. A smaller main jet will, therefore, richen the mixture. The inlet side of the main jet is identified by a stamped number, along with a chamfered edge, and should be installed inside the Primary By-Pass with both of those visible.

Installation of the Primary By-Pass Valve:
The body of the Primary By-Pass is marked with an arrow along with a #4. The arrow should point to the fuel tank or, for alcohol applications, to the inlet side of the fuel pump. For applications with long fuel lines, such as door cars with the tank in the trunk, please consult the factory for recommendations. See the Installation Schematic for location of this valve in the fuel system.