Vacuum at Idle Question
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Elder User
Joined: Mar 2009
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Vacuum at Idle Question
I rebuit my one barrel carter this last weekend and put it on the truck. I started it and timed it to about 8 degrees. I messed with the Idle mixture until it seemed to be running right but I was under the impression that the best way to set the Idle Mixture was with a vacuum guage. What I found is that there is little or no vacuum at Idle. It only seems to be getting Vacuum when I bring the truck up off Idle. Can anyone shed some light on this? The truck seems to be very snappy off idle with no hesitation in acceleration. There is no black or blue smoke coming out the tailpipe and the truck starts very easily. I guess in other words its running great even without vacuum at idle. I am taking the vacuum readings from the base of the carb because I know there is a special port for the Vacuum advance. Any words of wisdom would be greatly appreciated.
JRB65
JRB65
Senior User
Joined: Apr 2005
Posts: 446
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From: South Bend, IN
Sounds like you're either looking at ported vacuum (distributor advance). That source does increase with an increase of throttle opening, that's why it's used for ignition advance.
You want to be measuring vacuum at the distribution "tree" on the intake manifold. Trace back your brake booster or auto trans. modulator vacuum line (if you have an auto trans.) and measure your vacuum there. IF that engine is running that well you are going to have a lot better than 0 vacuum. Actually, even if the engine is running poorly you're going to have better than 0 vacuum, if it's running at all.
You want to be measuring vacuum at the distribution "tree" on the intake manifold. Trace back your brake booster or auto trans. modulator vacuum line (if you have an auto trans.) and measure your vacuum there. IF that engine is running that well you are going to have a lot better than 0 vacuum. Actually, even if the engine is running poorly you're going to have better than 0 vacuum, if it's running at all.
Cargo Master
Joined: Jan 2004
Posts: 2,512
Likes: 8
From: Cook Forest and Irwin PA
I rebuit my one barrel carter this last weekend and put it on the truck. I started it and timed it to about 8 degrees. I messed with the Idle mixture until it seemed to be running right but I was under the impression that the best way to set the Idle Mixture was with a vacuum guage. What I found is that there is little or no vacuum at Idle. It only seems to be getting Vacuum when I bring the truck up off Idle. Can anyone shed some light on this? The truck seems to be very snappy off idle with no hesitation in acceleration. There is no black or blue smoke coming out the tailpipe and the truck starts very easily. I guess in other words its running great even without vacuum at idle. I am taking the vacuum readings from the base of the carb because I know there is a special port for the Vacuum advance. Any words of wisdom would be greatly appreciated.
JRB65
JRB65
check manifold vacuum on the tree where the brake booster sucks from.
I pull 18.5 inches at idle
using the mix is the 'lean drop' method (google it up)
Thread Starter
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Elder User
Joined: Mar 2009
Posts: 520
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Thanks for all the insight on this vacuum issue, I am going over to work on it later today so I will try taking the reading off the Intake ports. The other day when I started it without the PCV valve I noticed a lot of vacuum on that line so Im sure Im getting vacuum, I am just taking the readings off the wrong ports. Again thanks and I will update this tonight after I take the readings.
JRB65
JRB65
