Hilborn Fuel Injectors

Technical Support - Mechanical - Fuel Injector Manifolds
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Related Tech Support Videos
Manifold Adjustments Video
Synchrometer Tuning Video

Manifold Adjustments

Hex Link Adjustment:
1. Remove injector stacks.
2. Remove all external throttle return springs.
3. Loosen lock nuts and back out idle stop screws on both ends of the injector.
4. Identify the hex link that runs to either side of the injector and the bronze throttle arms.
5. 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.
6. Reinstall throttle return springs.
7. Reinstall injector stacks.

Butterfly Arc Adjustment:
1. Remove injector stacks.
2. Remove all external throttle return springs.
3. Loosen lock nuts and back out idle stop screws on both ends of the injector.
4. 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.
5. 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 cor- rect, proceed to line 6. If gauge #2 is either loose or tight, see the selection below for further adjust- ments.
6. Remove gauging from the injector.
7. Reinstall throttle return springs.
8. Reinstall injector stacks.

Arc Adjustments:
1. Remove gauging from the injector.
2. Identify the hex link that runs to either side of the injector and the bronze throttle arms.
3. 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.
4. If gauge #2 is loose, lengthen the hex link up to a half turn.
5. If gauge #2 is tight, shorten the hex link up to a half turn.
6. Lightly tighten the lock nuts on the hex link.
7. 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.
8. 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 butter- fly against gauge #1, the arc is correct, proceed to line 10.
9. If gauge #2 is still out of specification, repeat all the steps until correct spacing is accomplished.
10. Without introducing bind in the heim joints, fully tighten hex link lock nuts.
11. Remove gauging from the injector.
12. Reinstall throttle return springs.
13. Reinstall injector stacks.

Centering Butterflies in an Injector:
1. Remove injector stacks.
2. Remove all external throttle return springs.
3. Loosen lock nuts and back out idle stop screws on both ends of the injector.
4. Identify bronze throttle shaft couplers, loosen both allen screws.
5. Identify the hex link that runs to either side of the injector and the bronze throttle arms.
6. Loosen the allen head pinch clamp screw on only one bronze throttle arm.
7. Loosen all the butterfly screws.
8. Lightly tap the butterflies closed with your finger while lightly sliding the throttle shaft back and forth against its stops.
9. Center throttle shaft side to side. Tighten butterfly screws using caution not to over tighten.
10. Tighten both allen screws on bronze throttle shaft coupler.
11. Perform same adjustments on other side of engine.
12. Tighten the pinch clamp screw on bronze throttle arm.
13. Reinstall throttle return springs.
14. Reinstall injector stacks.

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. For EFI applications, this step will ensure there is a good exhaust note and provide the best in drivability and engine acceleration. For Mechanical/Racing applications this step will ensure that all cylinders are at the correct temperature for the quickest front half times or optimal acceleration when exiting a turn. We have found that the use of a Synchrometer, 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 Synchrometer.
2. Place the Synchrometer on one of the throats and adjust the air flow restrictor so the needle is centered in the gauge. There is no correct starting place for the needle 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 air, adjust butterfly as to open it in the throttle bore, and conversely, if the cylinder requires less air, 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.