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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old December 30th, 2017, 03:15 AM
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Question Regarding vacuum advance and timing

Hello all,
I'm just trying to work out some discrepancies between what's in my maintenance manual and what I've found on the web and in some previous forum posts.

The first thing I've noticed is that the manual says timing should be 5 degrees BTDC. But, in digging through the forum I've noticed folks suggesting anywhere between 7.5 and 12 degrees?

The other thing that I can't make heads nor tails of is the manual doesn't mention anything about disconnecting the vacuum advance hose and plugging it as part of the process. But, pretty much everything I've found online and in the forums says that's a requirement?

I'm running a 305D BTW. Any guidance or pointers would be greatly appreciated as I'm still very much learning as I go.
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Old December 30th, 2017, 12:25 PM
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Default Re: Regarding vacuum advance and timing

In my 62 manual it states to disconnect the vacuum line (did not say to plug it) and set the 305D to 7-1/2° BTC. This information came out of the electrical section of the manual.

On my truck with a Stromberg carburetor (it only has one vacuum port), it is ported vacuum and not manifold vacuum. With the engine at it proper idle setting the throttle valve is not open enough to supply vacuum to the distributor.

With the vacuum line disconnected and the warm engine at proper idle speed does your line has vacuum on it? If it does not then increasing the engine rpm can you feel the vacuum? If this is the case then you have ported vacuum.

Manifold vacuum is there all the time no matter what the engine idle speed is set to.

The correct way to set the distributer is to disconnect the vacuum line and plug it. This way no matter which type of supply vacuum you have it will not affect the timing.

Last edited by James; December 30th, 2017 at 01:03 PM. Reason: Added more info
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Old December 30th, 2017, 01:04 PM
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Default Re: Regarding vacuum advance and timing

Perhaps that's the difference? Mine is a 61. From some of the other reading I've done, it sounds like the 60-61 years were quite similar..... and that changes started to come about after that?

I don't know the answer to your question about whether or not the vacuum is an 'always on' kind of thing. However, I'll definitely check..... The manual for my year definitely doesn't say anything about the vacuum advance though, and it states 5 degrees.

As always, I'm left with more questions and curiosity LOL
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Old January 2nd, 2018, 06:39 AM
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Default Re: Regarding vacuum advance and timing

My 60-61 manual also states 5 degrees btdc. I found my 305e was happiest with 8 initial without vacuum. Though with the eddy 4 barrel I have the choice of ported and manifold vacuum. I am using ported vacuum currently.
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Old January 4th, 2018, 01:36 AM
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Default Re: Regarding vacuum advance and timing

This is probably all old-hat to most of y'all. But, I found that the following article helped me to understand a bit better as to what's going on:

http://chevellestuff.net/tech/articl...r_manifold.htm
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Old January 4th, 2018, 02:58 AM
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Default Re: Regarding vacuum advance and timing

That an interesting article and I agree with it. However there is one engine configuration it does not touch on, it is deceleration comparison between manifold and ported vacuum.

If you are cruising down the highway and you take your foot off of the gas pedal, with manifold vacuum it can climb up to 21-25 " of vacuum. This would place the distributor can in full advance while decelerating. In my opinion (I could be wrong on this) you have a rich idle mixture (due to higher vacuum through a carburetor idle circuit) getting approximately an extra 15° (on top of whatever the centrifugal advance bring in) of ignition advance possibly causing the peak pressure to build before TDC and could be making the engine/car slow down faster. Could also contribute to washing the cylinder down with fuel (from the extra rich idle fuel mixture).

With ported vacuum under the same conditions the distributor can does not move and stays a 0°. Only the centrifugal advance is providing ignition advance base on rpm. This will could allow the engine/car to coast further.

Another point to bring up if you are using ported vacuum and you engine idle is set properly. Then you switch to manifold vacuum to supply your distributor can you may have an engine racing (idling fast) because you are not able to bring the idle back down where it belong. With an automatic transmission this is not acceptable.

I too have seem people disconnect their distributor can thinking they had more engine power. In reality they make more frequent stop for gas.

Disclaimer: The results you get may vary and you may or may not notice any differences.

Myself I always use ported vacuum, even in my hi performance cars/trucks because they are not for racing they are daily drivers.
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