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Old 08-07-2006, 05:20 PM
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vacume advance

on the fords i amworking on you set timing with vacume line off, inst line and you get some advance even with throttle plates closed as much as poss [ using holley 650]their is some signal,itis my understanding 1 gets advance via the vacuum as well as centrifical until near full throttle. the gm teck is saying toatle advance is not effected under full throtle on manifold vacume. on the ford ported signal it seems that it combines to give toatle advance as the vacume only drops off under heavy load as engine lugs. my question is am i missing something. iam useing a older vacume unit on my 302 with shims to delay vac advance as with latter sealed unit had to much advance part throttle. engine is in a 32ford infront of a c6. again the gm guy explained ther manifold system very well.any ways this is a good discusion as always i learn somethig cliff

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Old 08-07-2006, 05:39 PM
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On almost all vehicles you will disconnect the vac line at the distributor and plug it, even my Pinto was that way.

If you were setup as true ported vacuum you'd have no vac signal to the vac advance at idle, assuming throttle blades are adjusted correctly. That ported source would not be exposed the the pressure differential.

No matter the port location, manifold or ported, once you are under wide open throttle either location will give the same signal to the vac canister. The reason, simple, they both are looking at the same thing. Absolute pressure in the intake.

If your carb is a restriction then you can have a vacuum reading at wide open throttle. It may only be 2 or 3 inches of pressure drop but may be enough to pull in some of the vac advance.

Hooking a vacuum gauge to the manifold port on the carb and taping the gauge to the windshield is a good way to figure out what is going on.

I'm not quite clear yet though on what your saying the GM Tech guy was saying versus the way you're thinking the 302 is setup.
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Old 08-07-2006, 07:29 PM
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WOW, look what I started.

I think there are a few posts towards the beginning of this thread that just do not make sense. And that article by the GM tech is exactly why I am looking for manifold vacuum, and not ported. I have a vacuum gauge in the car and have double checked each vacuum port. I DO NOT have a manifold vacuum port on my carb ... which confirms what Holley said in the instructions.

I have since learned I can actually tap into one of the threaded holes on the intake and run a vacuum line off of it. I may try it that way.
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Old 08-07-2006, 08:56 PM
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Just don't tap off the PCV hose.
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