Vacuum Required for Vacuum Advance - The 1947 - Present Chevrolet & GMC Truck Message Board Network

4tiresngas · 11-03-2005, 08:08 PM

In another thread I described how my truck bogs down when I floor it. I thought I would check the vacuum where the vacuum advance on the distibutor is hooked up.

On the side vacuum port on my Q-jet, the vacuum gauge read around 15-20 while idle. When I pushed the accelerator, the vacuum dropped to 0. On the front port on the Q-jet, the vacuum read 0 at idle and went to 20 when I accelerated?

Is this right? and which one should the vacuum advance be hooked to?

Thanks.

dwaite72lnghrn · 11-03-2005, 08:31 PM

Normal vacuum hookup on a qjet is full vacuum 20" at idle stock engine. This puts about 16* advance in the engine timing. If the vacuum is lower at idle 15" engine is worn, needs tunedup or has a big cam. Ported vacuum has no advance at idle 0" and as the carb plates open it will jump to about 20" then as you start to move the vacuum will act the same as the full vacuum port. I always hook VA to full vacuum. Some people hook to ported with HEI dizzy. personal choice after running both ways I guess.
The gauge should be steady at idle, If it is bouncing then you have problems with the engine.

Cat Fuzz · 11-04-2005, 02:14 AM

You want to connect your vacuum advance to the port that has no vacuum at idle and 20" at open throttle.

bejay · 11-04-2005, 08:11 AM

you could hook it to either one it was probably originally hooked to ported vacum but hooking it to base or the one with vacum at idle offers some advantages such as better idle less chance of dieseling due to the throttle plates being closed further and it is commonly hooked to this port in later gm models in the late 70s and 80s.
dont suspect that it has anything to do with your problem as neither vacum port will have any vacum when you floor the accelerator or have the throttle opened very much.

11-03-2005, 08:08 PM	#1
4tiresngas An American Soldier Join Date: Feb 2005 Location: Grumolo D' Abadessee, Italy Posts: 503	Vacuum Required for Vacuum Advance In another thread I described how my truck bogs down when I floor it. I thought I would check the vacuum where the vacuum advance on the distibutor is hooked up. On the side vacuum port on my Q-jet, the vacuum gauge read around 15-20 while idle. When I pushed the accelerator, the vacuum dropped to 0. On the front port on the Q-jet, the vacuum read 0 at idle and went to 20 when I accelerated? Is this right? and which one should the vacuum advance be hooked to? Thanks. __________________ Dan dan.brue@us.army.mil

11-03-2005, 08:31 PM	#2
dwaite72lnghrn Registered User Join Date: Jun 2002 Location: central pa Posts: 1,525	Re: Vacuum Required for Vacuum Advance Normal vacuum hookup on a qjet is full vacuum 20" at idle stock engine. This puts about 16* advance in the engine timing. If the vacuum is lower at idle 15" engine is worn, needs tunedup or has a big cam. Ported vacuum has no advance at idle 0" and as the carb plates open it will jump to about 20" then as you start to move the vacuum will act the same as the full vacuum port. I always hook VA to full vacuum. Some people hook to ported with HEI dizzy. personal choice after running both ways I guess. The gauge should be steady at idle, If it is bouncing then you have problems with the engine. __________________ 72 longhorn c30 502BB/th400/fact air/4.11rear/ custom camper 72 bug(the better half's) under reconstruction 2009 HHR aqua blue 71 GMC k20 350/sm465 2009 Chevy hd3500 6.6 Victory Red crew cab dually Dave & Jeanne

11-04-2005, 02:14 AM	#3
Cat Fuzz My son, my joy. Join Date: Sep 2003 Location: Spokane WA Posts: 463	Re: Vacuum Required for Vacuum Advance You want to connect your vacuum advance to the port that has no vacuum at idle and 20" at open throttle. __________________ '69 Chevy 3/4 ton, 327cid, Carter AFB, Edelbrock intake, HEI, TH400. Founding member of the No Money To Spend On My Truck Club. When your at Home Depot, you WISH you had gotten an 8 foot bed!

11-04-2005, 08:11 AM	#4
bejay Registered User Join Date: Aug 2005 Location: missouri Posts: 136	Re: Vacuum Required for Vacuum Advance you could hook it to either one it was probably originally hooked to ported vacum but hooking it to base or the one with vacum at idle offers some advantages such as better idle less chance of dieseling due to the throttle plates being closed further and it is commonly hooked to this port in later gm models in the late 70s and 80s. dont suspect that it has anything to do with your problem as neither vacum port will have any vacum when you floor the accelerator or have the throttle opened very much.



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