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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: OldGray on September 17, 2016, 11:34:06 am

Title: Vacuum question - distributor, etc
Post by: OldGray on September 17, 2016, 11:34:06 am
My '79 was cobbled together in a way that has had me baffled, and left sorting out getting everything hooked back up the way it should be.  I have been trying to find a good basic vacuum diagram for my truck but haven't really found what I need yet. 

Does the distributor get full manifold vacuum?  Mine is hooked to the large output on the back of bottom of the throttle body (of my 2bbl Rochester).  But a diagram I found shows a "metered" (?) port labeled as the one to go to the distributor.  (see pic below)  So is the distributor supposed to get MORE vacuum under acceleration, or LESS? 

And does anyone know of a diagram I can use to get everything hooked back up the way it is supposed to be?

(https://lh3.googleusercontent.com/0VP41CS5XwjSJsVaR7jBMaWNuM_6UWqz2oqKFM-uiLVe-nnMAULmP78LQT6yQFbSr2sdrresgr9jH_eQUmXdX0HrJU-KrvCHZzYnQ2JtSCFLtryC6PBV3PqK_qRWFkQWb7jwKPkJ4romT_zMPYQ0az1NT2EjFfP_PnbH8ypW8zCIDvi2JbURNau6nvrAC-EWCOjd0TIY92t8Ue-df8d_8N9danZ5kwjH-hyYGsYA8bU7ZB4ELnVfR3n2-fxA3agwmsWTrOmEZCpwqb7YTiZbiGHwNIu-ofNxfy_h8j1ScX0amM9YyOqfRUAT6ZaNkbO5NxYutdlvWta54AJ56eJeKYrVv4Bn1_4OxS-KOSTpoHEhtRVQl7iiFM6096yE0FNpe2hQtATbgUu_KwEbEbIzQCvSVCCHCLKOxe4qR2sPTEnXZeQU_6c_sP2JXdt9Lgm_0PHw4xb7PB7BG7DKMcOs4s3cU9O05OZvDXz8xPUYXFyZB9LFtDOo0S_iOh_R27jRueGcmFb6iL0hoYDnNRe2sOiBfU5qvEjeo0HwzabHxV3VAPBWjjOWIxOkeXeUJdsLjMfHas8X8ZEi7m9g6TRjVBTTonLB8IUd5uI_54sGQf0YK61o=w511-h241-no)

THANKS

Title: Re: Vacuum question - distributor, etc
Post by: Stewart G Griffin on September 17, 2016, 04:11:53 pm
My 78 malibu 305 engine with the 2G carb (which i'm thinking should be similar to a 79) had the vac advance hooked up to ported as in the diagram.

Will try to find a pic asap.
Title: Re: Vacuum question - distributor, etc
Post by: OldGray on September 17, 2016, 04:42:43 pm
Heh heh - my buddy sent me a link to an article about how on an Automatic it is generally hooked up to manifold vacuum, and on standard it is metered.  And the info I find on the internet is back-n-forth on the subject. 

I even hooked it up to the metered vacuum earlier and couldn't really tell a difference on a short drive.  Weird.

However, I will admit that it has been hooked up to manifold vacuum since I got it and I haven't noticed anything weird.  It IS possible that the only thing weird about it is the WAY they hooked it up.

BTW - My truck came with a 350.  The motor that is in it is a rebuilt 305.  But the Rochester 2GV on it has numbers that say that it was from a 1970 Chevelle with a 350... so it looks like my truck is a mish-mash built out of parts laying around some guy's driveway!   :o

BTW - I LOVE how this forum censors even the MILDEST of words... Even ones that someone would hear in a church on a regular basis.  Thanks for protecting my delicate ears and sensibilities.  Whiskey-tango-foxtrot?

Title: Re: Vacuum question - distributor, etc
Post by: OldGray on September 17, 2016, 06:55:45 pm
I just spoke to two buddies who both have plenty of Chevy experience, and they both said, "Yup, chevy uses a manifold vacuum which advances the timing at idle".

So, jeepers creepers (am I allowed to say jeepers creepers?) I guess the thing is is the right gol-durned place.

THANKS
Title: Re: Vacuum question - distributor, etc
Post by: rich weyand on September 17, 2016, 07:09:04 pm
This is a sticky post at the top of the forum and answers your question:
http://forum.73-87chevytrucks.com/smforum/index.php?topic=32085.0
Title: Re: Vacuum question - distributor, etc
Post by: Stewart G Griffin on September 17, 2016, 07:16:36 pm
Quote from: OldGray on September 17, 2016, 06:55:45 pm
I just spoke to two buddies who both have plenty of Chevy experience, and they both said, "Yup, chevy uses a manifold vacuum which advances the timing at idle".

So, jeepers creepers (am I allowed to say jeepers creepers?) I guess the thing is is the right gol-durned place.

THANKS

Not in all cases.....AND i would suspect not in MOST cases.  But i don't have time to investigate fully right now.
Title: Re: Vacuum question - distributor, etc
Post by: OldGray on September 17, 2016, 09:03:38 pm
That was a good read but it will take a while to fully sink in, and I am still not really sure (by that) if my distributor should be connected to manifold vacuum (as it has been) or other?  Or does it connect to both at a valve that switches between the two?  (wish I had taken auto shop in high school...)

My only real goal is to get it back to as close to stock setup as I can.
Title: Re: Vacuum question - distributor, etc
Post by: rich weyand on September 17, 2016, 10:18:03 pm
A 1979, as a matter of keeping it stock, should be connected to ported vacuum.  It was a pollution control measure.

Any Gen 1 engine should, as a matter of engineering, be connected to manifold vacuum, for best performance.
Title: Re: Vacuum question - distributor, etc
Post by: OldGray on September 17, 2016, 10:24:49 pm
Quote from: rich weyand on September 17, 2016, 10:18:03 pm
A 1979, as a matter of keeping it stock, should be connected to ported vacuum.  It was a pollution control measure.

Any Gen 1 engine should, as a matter of engineering, be connected to manifold vacuum, for best performance.

Ok - thank you!  That is a very condensed version of what both my buddies said.  I have it on manifold and it has performed well that way so far.
Title: Re: Vacuum question - distributor, etc
Post by: blazer74 on September 18, 2016, 12:42:38 am
I'd leave it on manifold vac .