66 vacuum leak

[Rob M.]

I am almost certain that my 66 327/300 has developed a vacuum leak. I think it most likely is associated with the rebuilt original carb (rebuilt a couple of years ago by MacNiesh). Despite many adjustments and tinkering, the carb has never worked well. Any suggestions on the best way to track down the vacuum leak?

Thanks,
Rob
39795

[Wayne W.]

Re: 66 vacuum leak

Often the gasket arrangement will be incorect. There should be a thin sheet metal gasket next to the carb, with a normal gasket under that to the manifold. If the metal gasket is not there or they are not installed correctly, it causes problems.

[Rob M.]

Re: 66 vacuum leak

Wayne,

The thin metal gasket is there and I think it's installed correctly (not sure how it wouldn't be). Is there something that I can use to test if the vacuum leak is coming from there - something that can be sprayed along the base? I want to determine that it's not an internal vacuum leak.

Thanks again,
Rob

[Paul G.]

Re: 66 vacuum leak

Rob,
Vacuum leaks are sometimes tough to locate. I've always used carb cleaner to spray around the carb, intake runners etc. with the car running. As you're spraying around if you hear the RPMs increase, you've likely found the source of your leak. Can you give some specific symptoms you're experiencing? Poor idle quality, idle trails off etc.

Paul

[Wayne K.]

Re: 66 vacuum leak

Rob,

I've never had to try it but I've heard of spraying carb cleaner in suspect areas or opening a propane torch holding the tip near the suspect areas and the engine should gain rpm when you find it.

Wayne

[Rob M.]

Re: 66 vacuum leak

Paul,

Thanks for the tip on the carb cleaner. The symptoms are: a high pitch whistle sound coming from the carb area and poor idle quality. The whistle is the new player in the game. As far as idle quality, I have never been able to get the idle set to spec. It's set at 700 now, but I can screw the idle adjust all the way in and it won't stall the engine. It has done that sense I got it back from the rebuilder.

Thanks again for the suggestions,
rob

[Rob A.]

Re: 66 vacuum leak

Rob,

I had the same high pitch whistle noise you mentioned on a '66 300HP car. It occured at warm temperatures and only at idle. That usually means the carb mounting surfaces (carb to base and base to intake) are warped. Ask MacNiesh if checking the carb surfaces for warpage and resurfacing as necessary was part of the rebuild process on your carb. Sometimes if the warping is minimal the whistle will go away for awhile with new gasgets installed, but it will come back.

[Rob M.]

Re: 66 vacuum leak

Rob,

Thanks and I think you could be right about a surface being warped. I will try to contact him and ask about it. Right now, I am tempted to just buy a brand new Holley and say the h*#! with it.

Rob

[Joe R.]

Re: 66 vacuum leak

I agree that the most likely cause of the whistle sound is a warped base plate. This is a common problem with Holley carbs.

As a temporary fix, some people install two gaskets stacked on top of each other, in order to get more crush thickness to make up for the warpage. However, this should be viewed as only a temporary measure, and it can actually increase the warpage if you apply too much torque to the mounting nuts.

Other possible sources of leakage (though rarely whistling) include the areas where the throttle shafts go through the base plate.

The good carb rebuilders check for warpage during a rebuild and resurface the base plate if necessary. They also check for excessive wear on the throttle shaft bushings.

If this is your original carb and you want to keep it on the car, you can have the base plate checked. Alternatively, it might be possible to purchase a new base plate assembly from Holley and transplant it to your original carb. I'm not sure if the current replacement is identical in appearance to the original, though.