Hilborn Mechanical Fuel Injection Overview

10 min read

Hilborn Mechanical Fuel Injection Overview

10 min read

A mechanical fuel injection system from Hilborn includes the manifold, nozzles, barrel valve, and hoses. The main jet, housed in the primary bypass valve, is also included along with 2 additional jets, or pills. To complete the system, you will need ram tubes, fuel pump, fuel filter, fuel shutoff valve and, possibly, a hi-speed bypass valve. If Hilborn cannot supply a fuel pump drive, you will need to fashion a drive for the pump to run at 1/2 crankshaft speed.

Using your supplied engines specs, racing venue, car characteristics, etc., we will flow the entire system. This provides a very good starting point but tuning will be required to optimize the injection system. A mechanical system is really quite simple, but needs to be tuned to the current weather conditions.

There are 4 main parts to a mechanical system:

  1. Fuel Pump
  2. Fuel Nozzles
  3. Main Jet (or pill)
  4. Barrel Valve

Since this is a constant flow system, fuel is supplied to the engine continuously. As the pump spins, the restriction in the system (set by the nozzle and main jet) develops pressure and, therefore, fuel flow to the engine. The nozzle size is picked to deliver enough fuel at WOT for your engine, while the main jet is used to adjust the amount of fuel allowed to return to the tank, setting the actual fuel curve. This is exactly opposite of a carburetor. The barrel valve acts like a fuel shutoff, controlling the amount of fuel for idle and transitional fuel to WOT. The barrel valve also works in conjunction with the secondary bypass valve for better part throttle fuel control. At higher RPM the fuel pump will produce more pressure and volume than the engine can use, so a hi-speed bypass (or cutoff) is required.

Operating Tips

These operating tips and suggestions are the result of experience gained by years of running under varied conditions.

  • Due to the extreme clean-burning of finely atomized fuel, spark plugs generally take a long time to show any color. Do not change the mixture because of plug readings based only on a few laps. Pay more attention to the indications on the tops of the pistons, if visible, than to the plugs.
  • If the system does not operate correctly, disconnect the nozzle hoses, remove nozzle caps and blow through the nozzles with air pressure. If this procedure does not solve the problem, we recommend sending the injector, fuel pump and by-pass valve to our factory for testing and/or repair. When returning the unit, please supply all pertinent information such as the cubic inch of the engine, the type of fuel being used (gas, alcohol, or percentage of nitro) and manifold pressure if supercharged.
  • Do not allow running low levels of fuel in the tank as it will wash away from the pickup in the tank when the car is in the turns. The resulting gulp of air sucked into the system will give a momentary misfire in the engine. This is the first warning when running low on fuel.
  • Avoid tampering with the adjustments continuously. The unit is factory set and ready to run as received. Simply install the injector, select the desired starting by-pass jet and go out and race.
  • Shut the fuel off when the engine is not running to keep the nozzles from dripping. A shut-off valve should be installed on the outlet side of the fuel pump.
  • When the car has been left standing with the engine hot, you may notice that the throttle has become a little tight. This is due to expansion of the aluminum casting as it picks up heat from the engine. When the engine is started, the fuel spray cools it down and the throttle will free up immediately.
  • Extreme care should be taken to keep dirt from getting past the fuel filter and plugging a nozzle. Whenever the fuel lines are disconnected, tape up the openings immediately. Also watch for dirt in the threads of fittings when installing the pump and filter. Too much emphasis cannot be placed on the need for cleanliness.

How to set the metering valve

Set injector to wide open throttle. As pictured in the Metering Valve Schematic, the distance “X” between the center of the rod-end bearings is equal to the distance “Y” between the center of the throttle shaft and the center of the metering valve rotor (#54 shown). Note that the inlet fitting on the block is off-set to one side. Adjust the rotor so that the number is located on the wide side of the off-set. Tighten arm on the rotor and the throttle shaft.


Staff Writer
1715 Posts