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Hilborn Manifold Adjustments

Author: Hilborn | 11/17/2019 < Back to Motor Life Home

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 be adjusted before initial startup and when warm.


Adjustments should be made in the following order:


  1. Centering Butterflies
  2. Hex Link Adjustments
  3. Butterfly Arc and 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 ensuring that the butterflies are in a fully closed position for the arc adjustments and 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 on one side of the engine
  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, then tighten butterfly screws using caution not to over tighten
  7. Perform same adjustments on other side of engine
  8. Tighten the pinch clamp screw on bronze throttle arm


Centering Butterflies (Blower Hat or Scoop Injectors)


This operation verifies the butterflies are centered in the bores, which removes the risk of sticking along with ensuring that the butterflies are in a fully closed position for the arc adjustments and 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. If your injector is electronic, 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, then tighten butterfly screws using caution not to over tighten


Hex Link Adjustment


This adjustment verifies the hex link, along with the bronze arms, is at the optimal starting point for the butterfly arc adjustments.


  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 starting hex link length. Note: The ideal starting length of the hex link is to make the centers of the heim joints the same center dimension as the butterfly shafts. An eyeball adjustment is all that is required at this time. NOTE: One side of the hex link will have a groove around the outside of the hex link; this to identify a left hand thread.
  4. Verify correct bronze arm angles by opening the throttle from idle to wide open, making sure the tip of the bronze arms are split on either side of 12 o’clock with relation to the butterfly shafts e.g. 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.
  5. Verify that both sets of butterflies are in the fully closed position by loosening one bronze throttle arm, allowing the butterflies to fully close, then tighten the pinch clamp screw and begin butterfly arc adjustments.


Butterfly Arc and Adjustments


Ensuring 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, and will result in a rich/lean condition on each side of the engine and poor drivability.


  1. 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.
  2. Insert gauge #1 between the center of the throttle blade and wall of injector making sure that the gauging material stays in contact with the machined portion of the throat. 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, remove gauging 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. Identify the hex link that runs to either side of the injector and the bronze throttle arms
  3. Loosen lock nuts on both sides of the hex
  4. Either lengthen or shorten hex link 1/4 to 1/2 of a turn
  5. Lightly tighten the lock nuts on the hex link
  6. Loosen the allen head pinch clamp screw on only one bronze throttle arm, lightly tap the butterflies closed on either side of the injector and tighten the pinch clamp screw
  7. 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 9.
  8. If gauge #2 is closer to the ideal drag, repeat steps 4 thru 8 until correct spacing is accomplished. If gauge #2 identifies you are moving away from the ideal drag, reverse the direction you are turning the hex link and repeat steps 4 thru 8.
  9. 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. 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 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.
  11. You have now completed the injector manifold adjustments. Install your ram tubes and enjoy your Hilborn Fuel Injector.
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