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Old 12-04-2019, 11:59 PM   #16
PC/SR
Senior Member
 
Join Date: May 2010
Location: Santa Rosa, CA
Posts: 1,276
Default Re: Vacuum testing to diagnose fast idle

You are never going to get a rock steady vacuum reading on a Model A at idle. It should steady up as speed increases. Try the gauge at max retard. Using a tachometer with a vacuum gauge helps with diagnosis. Loose valve guides means an air leak through probably worn valve guides.

That said, the low compression reading on #2 is worthy of investigation. I could be a lot of things but lash is the first and easiest thing to check as noted. I personally doubt that low compression in one cylinder is going to have a major effect on idle, although it could cause a mis-fire.

Sounds too simple, but have you checked manifold torque, and/or gasket leak? Easy to check, and I like to check the easy things first. It is too often that we over-analyze problems.

I personally think it is a carb problem. (Or ignition as second choice.) The best/easiest way to check/eliminate carb is to swap your carb with a pal's carb that runs right, and try your carb on his car and see if you can adjust it.
Troubleshooting is an art. Trace one system from start to finish instead of guessing. Any symptom can have multiple causes which must be eliminated. For example you might even have an obstructed muffler. These things can get wacky. Back in the day, before computers, a good troubleshooter was golden in any shop. But look at it this way--you are going to learn alot about your car and that is a large part of the fun of the hobby.

Last edited by PC/SR; 12-05-2019 at 12:09 AM.
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