Questions and Answers
Low Chevy Vacuum
Q. Dear Vincent: I have been struggling with a backfiring problem on my rebuilt 1962 Chevy 327 that has a 600 cfm Edlebrock carburetor. I think I may have a vacuum problem and have not been able to determine what the manifold vacuum should be on this engine. It registers about 12 inches at idle and will increase to about 18 inches as the idle increase to about 1200 RPM. Can you help?
Thanks,
RogerA. I would almost have to say that you have a tight valve somewhere. The general rule of thumb is low rpm, high vacuum. High rpm, low vacuum. If you have ever seen a fuel economy gauge, you will see at high rpm it shows low fuel milage and at low rpm, it shows high milage. Well, all that gauge is is a vacuum gauge and it's measuring intake manifold vacuum.
If the needle on the vacuum gauge is fluctuating, then you have a tight valve. If it's low and steady, then the problem is with the timing. In fact, a lot of old timers would set the timing with a vacuum gauge. When it got to the highest reading, they locked the distributor right there.
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