What would cause low vacuum?
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Tuned
Joined: Apr 2011
Posts: 298
Likes: 4
From: Highland CA
What would cause low vacuum?
Hi guys I went to a carburetor shop today to have my Holley 2bbl LIST # 7501.
Fuel/air mixture adjusted, The mechanic guy said vacuum is low at 15Hg is this bad,He said 18Hg to 20Hg is normal, What would cause low vacuum. I have a stock engine,cam,lifters,everything is stock, He also said that the idle screw , out is lean/ in is rich on this Holley 2bbl,
I thought it was the opposite in is lean, out is rich.
any advice is appreciated thanks.
Fuel/air mixture adjusted, The mechanic guy said vacuum is low at 15Hg is this bad,He said 18Hg to 20Hg is normal, What would cause low vacuum. I have a stock engine,cam,lifters,everything is stock, He also said that the idle screw , out is lean/ in is rich on this Holley 2bbl,
I thought it was the opposite in is lean, out is rich.
any advice is appreciated thanks.
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Joined: Dec 2001
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From: Louisville,Ky.
Some Holley carbs use a reverse idle circuit in which that is true. Not sure about that list number though.
The items listed above are most common. I would start with the base timing first. I like to run 10 to 12* btdc with no vacuum to the advance, and the advance should be ported (no vacuum at idle)
The items listed above are most common. I would start with the base timing first. I like to run 10 to 12* btdc with no vacuum to the advance, and the advance should be ported (no vacuum at idle)
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Tuned
Joined: Apr 2011
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From: Highland CA
http://oldholden.com/ohfiles/Holley%...t%207508-1.pdf
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Tuned
Joined: Apr 2011
Posts: 298
Likes: 4
From: Highland CA
This is some info I found on the Holley 2bbl carb list # 7508-1
http://oldholden.com/ohfiles/Holley%...t%207508-1.pdf
http://oldholden.com/ohfiles/Holley%...t%207508-1.pdf
At 20*BTDC 15Hg is the highest vacuum I can get,anything less than 20*BTDC vac drops,my compression is 135psi all 8 cylinders.
Cross-Country
Joined: Jul 2007
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Low vacuum can be caused by alot of things; Is the gauge doing anything else. look at the vacuum gauge reading chart on this website
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Joined: Feb 2013
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From: constantine,mi.
Holley carbs that have backward adjustments have a label CC on them [counter clockwise] near the idle mixture screw
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Tuned
Joined: Apr 2011
Posts: 298
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From: Highland CA
http://oldholden.com/ohfiles/Holley%...t%207508-1.pdf
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