Just wondering how these work, as they won't seem to open on my truck. Stock "made for Motorcraft by Holley" carb.

I could only look while at idle while under the hood, but when I pushed the throttle all the way open (obviously for a short time, as it revs quite quickly), I did not see the secondaries budge. Additionally there is a thin layer of varnish on the secondary throttle plates; looked completely different than the plates on the primaries which are whistle clean.

Forgive my carburetor-ignorance, I know EFI inside and out, but this is my first time wrenching on a carb'd vehicle.

How do these work, and do they ever fail? What should be done, etc..? TIA!

 

There is a small passage from the diaphragm housing on the pass side rear of the carb, through the carb body and opens to the throat area in the carb. As air flows through the carb, it creates a lower pressure. Somewhat like a vacuum but not as dramatic. This lower pressure starts acting on the diaphragm that the secondaries are hooked to on the pass side rear of the carb, and they open up.

If you open the throttle with your hand about half-way, you then should be able to reach around to the pass side rear of the carb and push up on the lever that goes to the diaphragm for the secondaries, and they will open manually(do this with the engine off).

You will never get them to open sitting in the driveway. You have to work the engine hard to get them to open. Also, if you are running a chrome open element aircleaner, that will also delay the opening of the secondaries. The chrome aircleaner flows better than the stock cleaner, so the pressure in the carb throat is not as low as it would be with the stock aircleaner.

 

Yes, the secondaries are vacuum actuated.

Before I rebuilt my carb they would only open grudgingly at 3200 RPM, however now I've noticed them coming in at 2,500 R's under heavy throttle.