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jpsage 03-15-2002 10:33 AM

Chevy SB vacuum advance
I have a '72 Chevy truck, stock. I recently had it worked on and it now runs poorly. It pings under heavy accel. It feels like the advance curve is messed up. Visual inspection has me confused. The vacuum advance is connected to the service that provides no advance at idle (i.e. when the butterfly is closed). I always thought that you would have vacuum on the distributor except when engine was under full load. Should the vacuum advance be connected directly to the intake manifold? If someone could point me to any good books or web pages on engine timing vs RPM vs load I would be thankful.

dmorris1200 03-15-2002 04:36 PM

I'm assuming it's probably a small block V-8 in your truck. And I also think they were still using points in 1972. If that's the case first I would check dwell to make sure points are set correctly(a visual inspection of them is not a bad idea either), I think it should be around 30 degrees but I don't have specs handy. When your dwell is set then you need to check your base timing by disconnecting vacuum to distributor and checking timing marks with timing light. I think you should be around 4 or 6 deg. BTDC. If all that's good and it's still pinging you might want to get specs and check what total advance for your application should be. Also get some fresh gas, maybe you got some bad gas(happens). And I'm pretty sure your vacuum hose goes to ported vacuum(like you described), at least it does on all my Fords.

jpsage 04-01-2002 07:32 AM

Thanks for the input. I got out my trusty old timing light and found that the static advance was around 16BTDC. I spun the engine up and saw that the mechanical advance was working. I don't know how a mechanic with an analyzer could have messed it up that much. Oh well, it is running much better. Thanks for your help. OBTW, I repurchased "How to Hotrod Small Block Chevy Engines". It is a real good book that I had back in the 80s. Maybe I will get the old gal a new dress and add a few ponies. Thanks again.

dmorris1200 04-01-2002 08:05 AM

Glad to here she's running better. You can probably play with that timing a little bit. usually you can advance a little more than factory specs and be ok. Just go two degrees at a time, keeping track of your changes, and see where she runs best at. :)

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