How to Adjust a Holley Marine Carburetor

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The Holley marine carburetor can be either a 2 or 4-barrel carburetor, depending on your motor, and will have the same number of idle mixture adjustment screws. Adjusting the Holley carburetor can be done in just a few minutes, but the screws must all be adjusted to the same RPM level on the vacuum gauge. Adjusting the carburetor to the highest RPM level possible while still maintaining a smooth idle will allow you to have better engine performance and fuel economy.

Items you will need

  • Screwdriver

  • Vacuum gauge

Start the Holley marine engine and let it warm up for about five minutes. Shut off the engine, and remove the rubber vacuum hose from the vacuum port at the base of the carburetor.

Attach a vacuum gauge to the vacuum port on the bottom of the carburetor. Turn the adjustment screws on the carburetor clockwise with a screwdriver until they are seated completely without over-tightening.

Turn all the screws counterclockwise one complete turn and start the motor again. Adjust each screw, alternating between each one and turning them one quarter turn clockwise until you reach the highest rpm on the vacuum gauge with the engine running smoothly.

Adjust each screw until they are all running smoothly at the same RPM. Shut the engine off and remove the vacuum gauge.

Attach the rubber vacuum hose to the base of the Holley marine carburetor.


  • Be careful of the hot engine parts when adjusting the Holley marine carburetor while the engine is running.
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