Vacuum advance not working? - Chevelle Tech
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post #1 of 6 (permalink) Old Apr 5th, 10, 12:07 PM Thread Starter
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Vacuum advance not working?

I was doing a routine tune up on my 402 Elky this weekend when I discovered something is not right with the ignition timing. When I set per the factory spec (10BTDC) it idles fine - but it spits and bucks and is a dog on WOT. When I advance the timing to 14BTDC, it runs strong as it always has during WOT.

When I remove the vacuum hose to the vacuum advance, the engine idle doesn't change at all. Can someone tell me if this is normal? I would think the engine RPM would change if this was working correctly. Lastly, can someone explain to me how the vacuum advance works in concert with the advance weights in the distributer? It seems like they do the same thing.

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post #2 of 6 (permalink) Old Apr 5th, 10, 6:23 PM
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Re: Vacuum advance not working?

You should be able to detect a change in RPM. To be sure check the timing with the vacuum advance hooked up, should be way off the end of the pointer. Could be the advance mechanism is froze up or the canister is leaking. I had to replace my advance canister last year due to a leak.


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post #3 of 6 (permalink) Old Apr 5th, 10, 7:41 PM
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Re: Vacuum advance not working?

Is it hooked to manifold or ported vacuum? If hooked to ported it shouldn't have any effect at idle. Also, some canisters don't activate at idle if the vacuum isn't high enough. Be sure to check your total timing also as factory initial timing specs only apply if you're using an original distributor that still has original specs. Over the years it's very possible that the distributor has been replaced or modified.
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post #4 of 6 (permalink) Old Apr 5th, 10, 7:49 PM
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Re: Vacuum advance not working?

Seems like if the vacuum is hooked up to the right place on the carb, and the idle is nice and low, say less than 800, you should notice no difference with vac advance hooked or not. If you do, the vacuum advance is set waaay too low, or broken. In other words, you shouldn't have enough vacuum to be pulling the vacuum advance open (providing vacuum advance) AT ALL when idling. Vacuum advance adds spark/timing advance, in addition to the distributor's mechanical advance, throughout the power band, in direct relationship to throttle position, based on the vacuum condition inside the carb/intake.

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post #5 of 6 (permalink) Old Apr 5th, 10, 7:53 PM
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Re: Vacuum advance not working?

A couple of points. One, if you have even a mild aftermarket cam I am not surprised one bit that it needs more ignition timing than the factory 10 degrees.

Two, if your vac advance was connected to ported vacuum, then it's not getting a vacuum signal at idle, which is normal. Disconnecting it at idle shouldn't make a difference at port vacuum.

Three, connecting to manifold vacuum instead of port will give the additional vacuum at idle you are looking for (and your engine seems to want at 10 degrees BTDC).

Vac advance and mechanical advance are not interrelated. Vac advance controls timing based on vacuum which is a proxy for engine load and throttle position. Mechanical advance controls timing in relation to engine speed. They both add more timing but for different reasons.


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post #6 of 6 (permalink) Old Apr 6th, 10, 12:41 PM
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Re: Vacuum advance not working?

Nobody seems to have caught one part of the OP's statement. He is complaining about a bad running issue at WOT. There should be no (or at least very little) vacuum at WOT, so vacuum advance is NOT in play. Mechanical advance would be the only thing that may be wrong at WOT.

Here are some steps to follow.
  1. Plug a vacuum gauge into the port on the carb where you normally run your advance from. Record the reading at idle. If it is 0", you may be using the ported vacuum port. If so, slightly increase the RPM just enough to expose the port to a vacuum signal.
  2. Tee the gauge inline with the hose connected to the vacuum canister. If the reading is different, you more than likely have a bad canister diaphragm.
  3. Take the cap off the distributor and make sure that the mechanical advance is not sticking. Sticking mechanical advance can cause problems at WOT.

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