abe72 Posted April 28, 2014 Share Posted April 28, 2014 Hello all,I have a question: How do I know if the vacuum advance is not working properly and if replaced what difference should I see? Thanks, Abe Quote Link to comment Share on other sites More sharing options...
stangeba Posted April 28, 2014 Share Posted April 28, 2014 I'm sure there are other ways but first if you have a vacuum gauge, see if you are getting vacuum to the can. If not, find the blockage (TCS system?) or the leak and fix it. Then remove the hose and using a vacuum pump, apply about 15 Inches and see if it holds. No pump, suck on it and see if it holds. If it holds, next attach a timing light, start the engine, apply vacuum and the timing will advance. if you need exact measurements I would suggest a good timing light that has adjustments built in to zero the timing then add 2 inches of vacuum, record timing, add 4 Inches, record, add 6" and so on. Good luck! Bruce Quote Link to comment Share on other sites More sharing options...
Patrick Sheean Posted April 29, 2014 Share Posted April 29, 2014 Or you could just pull the vacuum line off when the car is idling and if the RPM's drop it is working, put it back on the RPM's go back to normal, yup it is working. Bruce is a professional I'm a California Okie, so my way requires no fancy gauges or tools, Bruce's way is the correct way though. Quote Link to comment Share on other sites More sharing options...
stangeba Posted April 29, 2014 Share Posted April 29, 2014 If it is hooked up to "PORTED" vacuum it would not drop. Ported vacuum is zero at idle then goes up as the throttle plates are opened. The original TCS spark (part of the emissions system) only allows vacuum to flow when the transmission is in high gear or reverse (for testing) so it wouldn't lower the speed if that was still attached and working. I would bet it has been torn off and the hose bypasses all the switches. It's what we all did back then, me included! If interested here are some pictures of how I bypassed my 71 350 4 BBL. pics of my 71 carb and modified air cleaner Quote Link to comment Share on other sites More sharing options...
abe72 Posted May 1, 2014 Author Share Posted May 1, 2014 Thank you both for your help in answering my question. I will be changing the it this weekend. Thanks again Quote Link to comment Share on other sites More sharing options...
Murphy Posted May 1, 2014 Share Posted May 1, 2014 If I may, does this info only pertain with a points style distributor? Quote Link to comment Share on other sites More sharing options...
stangeba Posted May 1, 2014 Share Posted May 1, 2014 No, vacuum advance was still on early GM HEI distributors. The modern cars no longer use them since they are controlled by computers. Here is a link to an article about vacuum advance, my boss (and brother in-law) a retired GM Chevrolet engineer wrote, for his business, Langdon's Stovebolt Engine Parts, web site. vacuum-advance-and-why-you-want-it Quote Link to comment Share on other sites More sharing options...
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