Vacuum Advance Question
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Fleet Mechanic
Joined: Dec 2006
Posts: 1,338
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From: UPSTATE NY
Vacuum Advance Question
well i'm just having second thoughts about my vacuum advance situation. it is running on constant right now, which is the port on the right hand side of the carburetor. this makes for a real strong pup, but tough to start. When i switch it over to ported vacuum (no advance during idle) it starts real easy but runs like poop. so what is everyones opinion about which vacuum advance to use? i know if i put it on ported vacuum and then just advance it a bit accordingly it will be just fine, but what would the difference in performance be? i have a performance distributors dui (hei) and an edelbrock carb
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Fleet Mechanic
Joined: Dec 2006
Posts: 1,338
Likes: 6
From: UPSTATE NY
ok so during constant vacuum advance it likes to be at 8 degrees BTDC.
with it on ported vacuum it idles and starts up great, but pinging when i step on it.
i am thinking the ideal thing for me to do here is leave it on ported vacuum and just advance it a little bit to compensate. how much is the timing advanced when you step on the gas at ported vacuum? is it the same it would be on constant except idle? at what rpm does it begin to advance?
with it on ported vacuum it idles and starts up great, but pinging when i step on it.
i am thinking the ideal thing for me to do here is leave it on ported vacuum and just advance it a little bit to compensate. how much is the timing advanced when you step on the gas at ported vacuum? is it the same it would be on constant except idle? at what rpm does it begin to advance?
New User
Joined: Jan 2008
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Is this a NEW V/A ?
Old ?
Is the Vacuum advance mechanism actually moving freely ?
Many times they are seized or the diaphram is has broken.
More info please ?
Is you acclerator rod on the carb indexed correctly ?
Do you have an EGR installed ?
Old ?
Is the Vacuum advance mechanism actually moving freely ?
Many times they are seized or the diaphram is has broken.
More info please ?
Is you acclerator rod on the carb indexed correctly ?
Do you have an EGR installed ?
New User
Joined: Jan 2008
Posts: 0
Likes: 24
Not having an EGR can cause pinging.
That's only why I mentioned it.
IIRC,the TOTAL advance is somewhere around 38*
have you checked that with timing light yet ?
You may have a centrifugal advance problem from weak advance plate springs.
That's only why I mentioned it.
IIRC,the TOTAL advance is somewhere around 38*
have you checked that with timing light yet ?
You may have a centrifugal advance problem from weak advance plate springs.
Posting Guru
Joined: Nov 2005
Posts: 1,302
Likes: 0
From: Madison, SD
Unplug your vacuum advance, and check to see what rpm the mechanical starts coming in. I'm not real familiar with the DUI, but with my MSD I have an adjustable vacuum advance chamber and change the springs on the mechanical. If it starts good with the initial at 8, then rattles later it could be that your vacuum is still coming in when the centrifugal advance starts to come in causing to much advance too soon in the rpm. If this is the case, try the stiffer springs on the mechanical side.... with today's fuel, I limit the total to around 34 total....
