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Hilborn Bypass and Cut-Off Valves Overview

Author: Hilborn | 11/18/2018 < Back to Motor Life Home
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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, as an option for alcohol applications only, 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.


The Secondary By-Pass Valve


The Secondary By-Pass Valve is used primarily for applications that get off and on the throttle at high rpm, such as sprint cars or applications with very low stall speed convertors. If the throttle is lifted to an idle suddenly at high rpm, the pump will continue delivering large quantities of fuel since the engine rpm is still high. Without the Secondary By-Pass, one can experience stumbling or excessive fueling at part throttle. With the Secondary By-Pass, this unwanted fuel is returned to the tank, and the system maintains the preset pressure.

Much like the Primary By-Pass, the Secondary By-Pass uses a poppet and spring arrangement, along with shims to help set the required pressure. Tailoring the pressure is accomplished with different spring tensions and shim combinations. The greater the tension on the poppet valve, the higher the pressure in the system.


Since fuel flow is controlled mechanically by the metering valve, any open throttle position of 25 degrees or more will automatically shut off the Secondary By-Pass. This eliminates the possibility of a lean condition when the throttle is open.


Installation of the Secondary By-Pass Valve


The body of the Secondary By-Pass is marked with an arrow and the letter “S”. 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.


The Hi-Speed Cutoff Valve


The Hi-Speed Cutoff is an optional valve that controls the higher rpm fuel curve of alcohol fuel engines that exceed 6500 rpm.


There is a certain point where the fuel pump output exceeds the required fuel flow of the engine. This typically starts to happen at 6500 rpm. Increasing the size of the main jet to return this additional fuel will leave the fuel curve very lean in the lower rpm. The Hi-Speed Cutoff is designed to remove this additional fuel without affecting the lower rpm fuel curve. Without the Hi-Speed Cutoff, the engine will seem to “lay-over” and stop accelerating as engine rpm’s rev past the 6500 rpm range. Gasoline engines will attribute this to a lean condition but with alcohol the condition is reverse, indicating a rich condition. Using a Hi-Speed Cutoff corrects this high rpm rich condition by bypassing unwanted fuel to the tank, restoring correct engine acceleration.


The Hi-Speed Cutoff uses a poppet and spring arrangement, along with a combination of thick and thin shims to set the required pressure. Tailoring the pressure is accomplished with different shim combinations. The greater the tension on the poppet valve, the higher the pressure in the system. When changes to the main jet in the Primary By-Pass Valve are made, a corresponding change should be made to the Hi-Speed Cutoff. For example, if the main jet is richened .005 (smaller orifice), one thin shim should be added to the Hi-Speed Cutoff, while a thin shim should be removed for every .005 you lean out the main jet.


Installation of the Hi-Speed Cutoff Valve


The body of the Hi-Speed Cutoff is marked with an arrow and the letter “C”. It connects directly off the fuel pump with the arrow pointing toward the fuel tank. See the Installation Schematic for location of this valve in the fuel system.


Leakdown Metering Valve


  1. Connect the leak tester to the top of the metering valve block.
  2. If the metering block has a brass secondary by pass fitting, cap it off.
  3. Close throttle to idle.
  4. Plug in the leak tester and read leakage through metering block.


Leak Tester Instructions


The HILBORN leak tester is a cylinder leak tester as well as a fuel injector leak tester. Hoses and spark plug adapters are provided for both uses. The large #H6 hose attaches to the metering valve, while the smaller #H3 hose is for the nozzles.


To operate, install an inlet air fitting (not supplied) of your choice and supply the gauge with at least 80 psi. Turn back the regulator knob to ZERO on the gauge. Attach #H6 hose to the metering valve or spark plug adapter. Plug in a quick-disconnect adapter. Leakage will read on gauge.


NOTE: When checking the metering valve assembly, the “S” secondary by-pass valve must be capped off. Leakage is always read in the idle position

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