Hilborn Fuel Injectors

Technical Support - Electronic - Fuel Injector Manifolds
Return to Technical Support

Manifold Adjustments

Manifold Adjustments Video

For correct operation of your Hilborn Injector the manifold will need to be tuned. This includes linkage and butterfly adjustments. If the injector is removed from the engine after being tuned, the process will need to be repeated. Although injector linkage is preset at the factory, adjustments will need to be performed after installation. Three piece injectors by nature of their design, will need to adjusted before initial setup and when warm.

Adjustments should be made in the following order:

1. Centering Butterflies
2. Hex Link Adjustments
3. Butterfly Arc Adjustments
4. Advanced Tuning

The first three adjustments will be performed with the ram tubes, throttle linkage and return springs removed. After you are satisfied with these adjustments, reinstall the throttle linkage, return springs and set idle speed. Recalibration of the TPS may be required. The fourth and final adjustment will then be performed. All adjustments should be performed with the engine at operating temperature.

Centering Butterflies (Stack Injectors)
This operation verifies the butterflies are centered in the bore which removes the risk of sticking along with allowing the blades to close completely for adequate idle speed control.

1. Loosen lock nuts and back out idle stop screws on both ends of the injector.
2. Identify the hex link that runs to either side of the injector and the bronze throttle arms.
3. Loosen the allen head pinch clamp screw on only one bronze throttle arm.
4. Loosen all the butterfly screws.
5. Lightly tap the butterflies closed with your finger while lightly sliding the throttle shaft back and forth against its stops.
6. Center throttle shaft side to side. Tighten butterfly screws using caution not to over tighten.
7. Perform same adjustments on other side of engine.
8. Tighten pinch clamp screw on bronze throttle arm.

Centering Butterflies (Blower Hat or Scoop Injectors) :
This operation verifies the butterflies are centered in the bore which removes the risk of sticking along with allowing the blades to close completely for adequate idle speed control. No other adjustments are required for hat or scoop injectors.

1. Loosen lock nuts and back out idle stop screws on both ends of the injector.
2. Do not loosen the pinch clamps that control the TPS linkage or improper function of the TPS may result. If you have upset this factory adjustment, please call the EFI tech line.
3. Loosen all the butterfly screws.
4. Lightly tap the butterflies closed with your finger while lightly sliding the throttle shaft back and forth against its stops.
5. Center throttle shaft side to side. Tighten butterfly screws using caution not to over tighten.

Hex Link Adjustment:
This adjustment verifies that both sets of butterflies are at the same starting point for correct butterfly arc when opened.

1. Loosen lock nuts and back out idle stop screws on both ends of the injector.
2. Identify the hex link that runs to either side of the injector and the bronze throttle arms.
3. Verify hex link phasing by opening the throttle from idle to wide open, making sure it is split on either side of 12 o'clock. In other words, idle will be at 10 o'clock, half throttle at 12 o'clock, and wide open will be at 2 o'clock. If not, loosen the two pinch clamp screws on the bronze throttle arms and adjust as necessary.
4.Loosen the allen head pinch clamp screw on only one bronze throttle arm, lightly tap the butterflies on either side of the injector closed with your fingers and tighten the pinch clamp screw.

Butterfly Arc:
Insuring the butterflies on each bank are opening at the same rate is critical for correct throttle tip-in and part throttle drivability. The use of feeler gauges for adjusting the hex link for correct engine "tone" side to side will not correctly set the butterfly arc, so drivability issues may result.

1. Loosen lock nuts and back out idle stop screws on both ends of the injector.
2. Verify the initial adjustment of the hex link by making sure the center to center dimension of the two heim ends are the same center to center dimension as the two butterfly shafts. This can be accomplished by using a dial caliper or by opening the throttle to center the heim with one of the butterfly shafts, allowing you to "sight down" to verify length of the hex link. If the hex link requires adjustment, the above procedure "Hex Link Adjustment" will need to be performed again.
3. Two pieces of round stock are required for gauging. Verify the gauging that you will be using to be within .001 of an inch from each other. Two Philips head screw drivers from the same manufacture or socket extensions are ideal.
4. Insert gauge #1 between throttle blade and wall of injector. While supplying light pressure with your finger to hold the butterfly against the gauge, insert gauge #2 between an open butterfly on the other side of the engine. If gauge #2 has a light drag without opening the butterfly against gauge #1, the arc is correct and you can remove gauging from the injector and proceed to "Final Tuning". If gauge #2 is either loose or tight, see "Butterfly Arc Adjustments" for further adjustments.

Butterfly Arc Adjustments:
1. Remove gauging from the injector.
2. Loosen lock nuts and back out idle stop screws on both ends of the injector.
3. Identify the hex link that runs to either side of the injector and the bronze throttle arms.
4. Loosen lock nuts on either side of balance bar. One side of the balance bar will have a groove around the outside of the hex link; this is to identify a left hand thread.
5. If gauge #2 is loose, shorten the hex link up to a half turn.
6. If gauge #2 is tight, lengthen the hex link up to a half turn.
7. Lightly tighten the lock nuts on the hex link.
8. Loosen the allen head pinch clamp screw on only one bronze throttle arm, lightly tap the butterflies on either side of the injector closed with your fingers and tighten the pinch clamp screw.
9. Insert gauge #1 between butterfly and wall of injector. While supplying light pressure with your finger to hold the butterfly against the gauge, insert gauge #2 between an open butterfly on the other side of the engine. If gauge #2 has a light drag without opening the butterfly against gauge #1, the arc is correct, proceed to line 11.
10. If gauge #2 is still out of specification, repeat all the steps until correct spacing is accomplished.
11. Without introducing bind in the heim joints, fully tighten hex link lock nuts.

Final Tuning:
Final tuning, or balancing of the butterflies insures that each cylinder is performing the same work at idle and at part throttle. When accomplished, your engine will start and idle extremely well, along with having good exhaust tone and provide the best in drivability and engine acceleration. We have found that the use of a Uni-syn, such as the Edelbrock #4025, allows us to maximize adjustments with the least amount of time. Before getting started, attach return springs and set idle speed. The engine and injector should be at normal operating temperatures.

1. Injectors with pinch clamps for ram tube retention will need to have the horizontal and vertical cuts in the casting taped from the inside to insure correct reading of the Uni-syn.
2. Place the Uni-syn on one of the throats, verifying the measuring tube is as vertical as possible, and then adjust the air flow valve so the float is centered between one of the dividers in the measuring tube. There is no correct starting place for the float since we are looking for equal airflow, not an airflow number.
3. Verify averages are consistent bank to bank. If they are not, remove throttle linkage and return springs and back off idle stops. Loosen a gold arm for the hex link and, while supplying slight closing pressure to the bank with the higher air flow values, tighten the gold arm for the hex link. This will allow both banks to open the same amount and even the airflow from bank to bank. It may take a couple of attempts before you are satisfied.
4. Identify butterflies that need to be adjusted to provide equal air flow. Open butterflies and identify which end of the blade points up. Mark the up end with a marker. Butterflies are beveled and need to go back in with the correct orientation (see Butterfly Installation). Also, adjustments to the butterfly are typically done on the end that points up.
5. Remove the two screws and lock washers that hold the throttle blade, taking care not to drop them into the engine.
6. Open throttle shaft to wide open throttle while grasping the end of the throttle blade to remove it. Protect the throttle blade from vise damage by using a piece of aluminum on either side. A rod vise works very well also.
7. Using the throttle shaft witness marks as a guide, insert the butterfly between the aluminum, lining it up with the witness mark. The end of the butterfly to be adjusted should be sticking out of the vise and both throttle shaft witness marks should be hidden.
8. Lightly tap the blade in the direction required. If the cylinder requires more airr, adjust butterfly as to open it in the throttle bore, and conversely, if the cylinder requires less airr, adjust butterfly to close it in the bore. Remember that .010 of an inch is a lot.
9. Reinsert butterfly into the throttle shaft. Install and tighten the butterfly screws.
10. Repeat as necessary. The closer the values, the better your engine will perform.

You have now completed the injector manifold adjustments. Install your ram tubes and enjoy your Hilborn Fuel Injector.

Please contact our EFI Tech Line for assistance with any of these procedures