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Quick-Checking Holley Vacuum-Secondary Operation

Scott Crouse Nov 1, 2001

Step By Step

Look into the carburetor on the primary side and locate the small hole (arrow) in the body.

Open the primary throttle linkage. This will allow the secondaries to open under vacuum. Use an air nozzle to blow air over the small vacuum hole located in step (1). You may have to move the nozzle around a bit until you find the sweet spot. The secondaries should open when the air flows past the vacuum port. If the secondary blades don’t open, there is a problem. Your secondary passage may be blocked or the diaphragm may be damaged.

Check the diaphragm first by removing it from the carburetor housing and removing the four screws that mount the lid. Inspect the rubber, making sure it’s not torn or cracked.

If everything looks OK, check out the carburetor housing by locating the entrance and exit of the port (arrow). Make sure the little cork gasket is in place and properly seats the port.

Cruising down the highway, you come up on a slow-moving car. As you merge into the passing lane, you press the accelerator but your car doesn’t seem to pass as quickly as you think it should. Pondering the fact that you just purchased a new high-horsepower crate engine, passing a slow-moving car should be the least of your worries. You know the manufacturer of your engine is reputable, and the motor isn’t the reason for the loss of power. As you think back to the installation, you realize that you used an old Holley vacuum-secondary carburetor and it could be that those back two barrels aren’t opening up under acceleration. In this article we show you how to cure this problem without removing the carburetor.

The only tools you will need are compressed air and a high-flow air nozzle. Before starting any part of this procedure, make sure the engine is off and remains off until the entire test is over. You can do this test with the carb on or off the car, but be careful not to let anything fall into the engine. If any carburetor work is to be performed, we recommend removing it and covering the intake manifold opening. Begin by removing the entire air cleaner.

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