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 idle/air mixture adjusted, the mechanic guy said vacuum is low at 15Hg is this bad, 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, because I thought it was the opposite in is lean, out is rich.
any advice is appreciated thanks
any advice is appreciated thanks
Lead Driver
Joined: Aug 2003
Posts: 7,592
Likes: 10
From: **** hole San Jose ca.
Timing maybe need to be set. Dizzy Vac. advance maybe sucking air. Most guy never check these when doing a tune up. Tired old motor in need of a valve job..
just for starters..orich
just for starters..orich
Fleet Owner
Joined: Dec 2009
Posts: 20,678
Likes: 82
From: NorCal
Hi guys I went to a carburetor shop today to have my Holley 2BBl idle/air mixture adjusted, the mechanic guy said vacuum is low at 15Hg is this bad, 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, because I thought it was the opposite in is lean, out is rich.
any advice is appreciated thanks
any advice is appreciated thanks
As a great American once said: "Trust, but verify" - Ronald Reagan
Logistics Pro
Joined: Oct 2005
Posts: 3,785
Likes: 4
From: Mddl A MexCans
In is lean/ Out is rich
The process is simple as to the A/F mixture screw method. The screws are moved in to lean the mix (cutting the fuel to air ratio) & (out to add fuel to the A/F ratio) richening the mix. As the mix leans up you will see a rise in vacuum, richen a lower. Too far in will choke out the motor, too far out will result in a low vac rich burn. A good airtight engine should OEM setting without any faults to the system will be 17-23hg imo. "Assuming that all the parts are working correctly in harmony" Timing, vac advance diaphragm leak, break booster, etc... low vac.
diagnosis via web/posts is futile... hope this will help.
How to Use and Interpret a Vacuum Gauge
diagnosis via web/posts is futile... hope this will help.
How to Use and Interpret a Vacuum Gauge
Post Fiend
Joined: Jun 2003
Posts: 8,209
Likes: 12
From: Black Hills of SD
Yes, holleys can be backwards. Timing or a tired engine are the 2 biggest reasons for low vacuum. But some engines just carry more than others too. 15 is a bit low, was it the needle steady? I've had perfectly healthy engines that wouldn't make more than 17, and even though they say the range is 17-23, I have never seen an engine that could pull 23, or 22, few even that did 21.
BTW, get a vacuum gauge of your own, it is one of my most valuable tools and one of the first things I grab when diagnosing.
BTW, get a vacuum gauge of your own, it is one of my most valuable tools and one of the first things I grab when diagnosing.
Thread Starter
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Tuned
Joined: Apr 2011
Posts: 298
Likes: 4
From: Highland CA
Yes, holleys can be backwards. Timing or a tired engine are the 2 biggest reasons for low vacuum. But some engines just carry more than others too. 15 is a bit low, was it the needle steady? I've had perfectly healthy engines that wouldn't make more than 17, and even though they say the range is 17-23, I have never seen an engine that could pull 23, or 22, few even that did 21.
BTW, get a vacuum gauge of your own, it is one of my most valuable tools and one of the first things I grab when diagnosing.
BTW, get a vacuum gauge of your own, it is one of my most valuable tools and one of the first things I grab when diagnosing.
You know I watched the guy adjust the A/F and wen he turned them in nothing happened, but wen he turned them out it would start to stumble and thats wen he said this idle screws are opposite for adjustment this is the list #7508-1and this # under 0488.
this # is not listed in the holley website what I have found is that this is an oem carb for Fe's
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Tuned
Joined: Apr 2011
Posts: 298
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From: Highland CA
What 2V Holley is on the rig? Look up the carb specs/tech info (Visit Holley's tech site at Holley Performance Technical Service Department and look up the resource docs) and evaluate his claim on the truthmeter. If I found out he's lying, then he is not to be trusted EVER again with my business.
As a great American once said: "Trust, but verify" - Ronald Reagan
As a great American once said: "Trust, but verify" - Ronald Reagan
in is rich,
out is lean,
LIST # 7508-1
0488
This is an oem replacement carb, not a performance carb, he called it "ECONOMASTER",270 cfm with a reversed adjustment for A/F mixture, So I think I just found a shop I can trust sense the guy new what he was talking about
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Tuned
Joined: Apr 2011
Posts: 298
Likes: 4
From: Highland CA
HIO I called a holley tech because I could not find info on the web site and he confirmed what the mechanic said about the idle screws,
in is rich,
out is lean,
LIST # 7508-1
0488
This is an oem replacement carb, not a performance carb, he called it "ECONOMASTER",270 cfm with a reversed adjustment for A/F mixture, So I think I just found a shop I can trust sense the guy new what he was talking about.
in is rich,
out is lean,
LIST # 7508-1
0488
This is an oem replacement carb, not a performance carb, he called it "ECONOMASTER",270 cfm with a reversed adjustment for A/F mixture, So I think I just found a shop I can trust sense the guy new what he was talking about.
Thread Starter
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Tuned
Joined: Apr 2011
Posts: 298
Likes: 4
From: Highland CA
Its around 20* ,if I try to set 6* or 12* the vacuum drops, and if I go above 20* it starts to ping, what do you think about recurbing the distributor?
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.
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.
Thread Starter
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Tuned
Joined: Apr 2011
Posts: 298
Likes: 4
From: Highland CA
What 2V Holley is on the rig? Look up the carb specs/tech info (Visit Holley's tech site at Holley Performance Technical Service Department and look up the resource docs) and evaluate his claim on the truthmeter. If I found out he's lying, then he is not to be trusted EVER again with my business.
As a great American once said: "Trust, but verify" - Ronald Reagan
As a great American once said: "Trust, but verify" - Ronald Reagan
http://oldholden.com/ohfiles/Holley%...t%207508-1.pdf
Post Fiend
Joined: Jun 2003
Posts: 8,209
Likes: 12
From: Black Hills of SD
I am wondering a little about how far the throttle butterflys are open at idle, or if you have a vacuum leak. If it doesn't respond real well to idle speed adjustment, maybe there's something we're missing. If the throttle plates have to be adjusted (idle speed) too much open to keep it running, the vacuum would be low and the response to idle mixture would be less noticable.
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Tuned
Joined: Apr 2011
Posts: 298
Likes: 4
From: Highland CA
I am wondering a little about how far the throttle butterflys are open at idle, or if you have a vacuum leak. If it doesn't respond real well to idle speed adjustment, maybe there's something we're missing. If the throttle plates have to be adjusted (idle speed) too much open to keep it running, the vacuum would be low and the response to idle mixture would be less noticable.
