Carb Mixture
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 851
Likes: 10
From: Palm City, FL
Carb Mixture
Going to adjust my carb mixture. I have a vacuum gauge. Can I disconnect the vacuum from the distributor vac advance and use that to connect my gauge? If not, what can I disconnect and connect the gauge to?
The vacuum tube to the dist advance, is the size that will fit on my gauge, so if I can use that, please suggest another place that uses a similar size vacuum tube.
The vacuum tube to the dist advance, is the size that will fit on my gauge, so if I can use that, please suggest another place that uses a similar size vacuum tube.
Le Mans Master
Joined: Oct 2010
Posts: 6,900
Likes: 833
From: melbourne florida
2023 C3 of the Year Finalist - Modified
i would use a manifold vacuum port that is not the vacuum advance. you want the engine in as run condition that includes the advance being hooked up
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
I have a plastic "T" with a cap on it in the vacuum hose going to my vac can, when I want to read manifold vacuum, I just pull off the cap and plug in the vacuum gauge. Easy. BTW, this will only work if your vac can is connected to manifold vacuum, not ported vacuum. If you don't know what I'm talking about, click on the sticky at the top of the tech/performance section, entitled consolidated FAQ, then click on Info on Timing, everything you ever wanted to know about ignition timing.
Scott
Scott
Burning Brakes
Joined: Feb 2012
Posts: 925
Likes: 7
For the fun of it, you can put a long hose on the gauge and run it out the back of your L88 hood and into the interior. Being able to see the tach, you can change engine speeds and record the vacuum say at 500 rpm increments. You can then get an idea what the vacuum advance can is seeing and doing to the timing if you know what can is on the distributor.
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 851
Likes: 10
From: Palm City, FL
Does it matter what size vacuum tube I use? I have an extra capped off port on the manifold, but it is for a small tube... i.e. it would be about the same size tube as what goes to the dist advance. But again it is directly on the manifold. Would that suffice to measure vacuum when adjusting carb mix?
Racer
Joined: Feb 2013
Posts: 386
Likes: 3
From: Clifton Virginia
Does it matter what size vacuum tube I use? I have an extra capped off port on the manifold, but it is for a small tube... i.e. it would be about the same size tube as what goes to the dist advance. But again it is directly on the manifold. Would that suffice to measure vacuum when adjusting carb mix?
Burning Brakes
Joined: Feb 2012
Posts: 925
Likes: 7
The size doesn't matter as long as you have good tight connections and the tap you're using is on full manifold vacuum, meaning the actual source is under the throttle plates. If you're seeing 12" or so that is manifold vacuum, if it's 0 or 2", you're on a ported tap (even if it seems that the tap is located below the throttle) and you need to find a manifold tap. I too have a tee in the line to the distributor for a gauge and of course, I'm running manifold vacuum to the distributor.
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 851
Likes: 10
From: Palm City, FL
The maximum vaccum I was able to get adjusting the carb mixture was just under 15".
Is that a normal range?
Here is what I did...
Drove car up to operating temp
Noted the idle RPM @ 1000
Shut engine off
Connected vaccum gauge to intake vaccum.
Adjusted the mix screws in all the way, then out 1/2 turn, so I know they are both at the same adjustment
Turned car on
Adjusted both screws in and out in 1/8 turn increments looking to see when the Vaccum maximized.... maximized at just under 15", both screws adjusted the same.
Ajusted Idle to 750.
Drove car to make sure it felt right and that the idle was stable.
Is that a normal range?
Here is what I did...
Drove car up to operating temp
Noted the idle RPM @ 1000
Shut engine off
Connected vaccum gauge to intake vaccum.
Adjusted the mix screws in all the way, then out 1/2 turn, so I know they are both at the same adjustment
Turned car on
Adjusted both screws in and out in 1/8 turn increments looking to see when the Vaccum maximized.... maximized at just under 15", both screws adjusted the same.
Ajusted Idle to 750.
Drove car to make sure it felt right and that the idle was stable.
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Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
The maximum vaccum I was able to get adjusting the carb mixture was just under 15".
Is that a normal range?
Here is what I did...
Drove car up to operating temp
Noted the idle RPM @ 1000
Shut engine off
Connected vaccum gauge to intake vaccum.
Adjusted the mix screws in all the way, then out 1/2 turn, so I know they are both at the same adjustment
Turned car on
Adjusted both screws in and out in 1/8 turn increments looking to see when the Vaccum maximized.... maximized at just under 15", both screws adjusted the same.
Ajusted Idle to 750.
Drove car to make sure it felt right and that the idle was stable.
Is that a normal range?
Here is what I did...
Drove car up to operating temp
Noted the idle RPM @ 1000
Shut engine off
Connected vaccum gauge to intake vaccum.
Adjusted the mix screws in all the way, then out 1/2 turn, so I know they are both at the same adjustment
Turned car on
Adjusted both screws in and out in 1/8 turn increments looking to see when the Vaccum maximized.... maximized at just under 15", both screws adjusted the same.
Ajusted Idle to 750.
Drove car to make sure it felt right and that the idle was stable.
If your engine is stock, 20" Hg is about what you should expect to see. Do you have an aftermarket cam, bigger than stock? This can make idle vacuum lower, and is normal. However, a vacuum leak can also lower vacuum readings at idle, and is not normal.
Here is a good article on how to read a vacuum gauge:
http://www.secondchancegarage.com/public/186.cfm
Scott
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 851
Likes: 10
From: Palm City, FL
What did the idle speed adjustment do to the manifold vacuum reading? Anything? Nothing? When you adjust your idle screws, and then make an idle speed adjustment, you should go back and adjust the idle mixture screws again. Don't start from scratch, just adjust them from where they are now. Keep track of how many turns out you are. When you're happy with it, shut the engine off, and screw the idle screws all the way in, counting the turns, then back them out again, same number of turns. This serves two purposes: You have a baseline for future tuning, and it verifies that both screws are turned out the same amount.
If your engine is stock, 20" Hg is about what you should expect to see. Do you have an aftermarket cam, bigger than stock? This can make idle vacuum lower, and is normal. However, a vacuum leak can also lower vacuum readings at idle, and is not normal.
Here is a good article on how to read a vacuum gauge:
http://www.secondchancegarage.com/public/186.cfm
Scott
If your engine is stock, 20" Hg is about what you should expect to see. Do you have an aftermarket cam, bigger than stock? This can make idle vacuum lower, and is normal. However, a vacuum leak can also lower vacuum readings at idle, and is not normal.
Here is a good article on how to read a vacuum gauge:
http://www.secondchancegarage.com/public/186.cfm
Scott
http://forums.corvetteforum.com/c3-t...-magazine.html