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Carburetors Carb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.

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Old 07-10-2005, 08:10 PM   #1
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Tuning carb with vacuum gauge

Ok, i am new to the carb world and have a few questions. I bought a vacuum gauge so that i can tune the carb correctly. Is a vacuum gauge the right thing to use? Will it get my air fuel mixture correct? i have read a few articles that say when using the gauge to adjust the idle mixture screw untill the most vacuum is present, is this correct?
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Old 07-10-2005, 10:55 PM   #2
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After getting the most vacuum lean it out a about 1/8 turn on the idle mixture screw and you should be near your lowest emission point.
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Old 07-11-2005, 12:20 PM   #3
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ok, well i got it tuned and its reading around 10-12. this normal?
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Old 07-11-2005, 01:41 PM   #4
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No. On a stock engine 20" is around the ballpark.
What carb?
What's your altitude?

Here at 4000' I get around 17". What's your idle speed set to as well?
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Old 07-11-2005, 03:14 PM   #5
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carb is a edelbrock 600 (1406) Altitude im not sure of but i know that the local drag strip is at 1475 feet above see level, and thats just about 15 minutes down the road. As for the idle im not sure, my tach is way off, but i set the idle pretty low, as low as i could without it stalling
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Old 07-12-2005, 04:47 PM   #6
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Ok now let me ask you guys this, when i am tuning the carb with the vacuum gauge, i can get it to where it peaks, then i can adjust the screws about 2-2.5 turns and the needle doesnt move. I was driving it after i set the screws and i noticed it was running very rich, i could tell by the exhaust and my a/f ratio gauge was reading rich. Is this condition normal?
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Old 07-14-2005, 02:08 AM   #7
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Quote:
Originally posted by del91_305
Ok now let me ask you guys this, when i am tuning the carb with the vacuum gauge, i can get it to where it peaks, then i can adjust the screws about 2-2.5 turns and the needle doesnt move. I was driving it after i set the screws and i noticed it was running very rich, i could tell by the exhaust and my a/f ratio gauge was reading rich. Is this condition normal?
It's running rich because you turned it two to two and a half turns after the needle stop going up. You need to turn them back in and then strat turning them back out thell the needle stops riseing. Then turn it back in 1/8 of a turn. This should get you right where you need to be.

Also you should have around 10" at idle and 20" when you are crusing and 0" when you are 100% throttle. It verrys from cam specs but assuming that you have a stock cam this is where you should be, Probable a lil higher then that on the vacum but still zero when flooring it.
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Old 07-14-2005, 11:06 PM   #8
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Quote:
Originally posted by mrt89rs

Also you should have around 10" at idle and 20" when you are crusing and 0" when you are 100% throttle. It verrys from cam specs but assuming that you have a stock cam this is where you should be, Probable a lil higher then that on the vacum but still zero when flooring it.
My engines all idle with atleast 18 in/hg of vacuum with the stock cam. My 312 idles with 23 in/hg of vacuum. Something is wrong if you can only get 10 in/hg at an unloaded idle. My 312 will idle at 650 rpm and 16-17 in/hg in gear with the A/C and both blower on high. I think you have the readings for cruise and idle swapped. I see about 13 in/hg at a 75-80 mph cruise with the engine running about 1,800.
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Old 07-14-2005, 11:26 PM   #9
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You might have your vacuum gauge hooked up to port vacuum instead of manfold vacuume. It should be on manfold vacuum

Those reading are from my 305 with stock cam, 305 with 262 cam and 383 with 292 cam. all of them had semulor readings. My stock cam would pull 14 at idle with no load and 23 under load with low throttle, 262 cam would pull 10 at idle and 20 under load and low throttle, 292 cam pulls the same as the 262 both my 262 cam and my 292 cam had 110 lobe seperation.
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Old 07-15-2005, 01:34 AM   #10
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I read mine straight off the fitting on the back of the manifold. I have a vacuum gauge permenantly inserted into the dash. With the stock LE9 carbed 305 it made 22 in/hg @ 700 rpm in neutral, 20 in/hg in drive, 16 in drive with the A/C on, 12-16 during cruise, and 3-12 during acceleration. With the XE-274H it made 15 in/hg @ 750 in neutral, 14 in drive @ 700, 12 in drive with the A/C on, around 12-16 during cruise and 0-8 during acceleration. My 355 with the LT1 cam would make 24 @ 550 in neutral, 21 in drive, 19 in Drive with the A/C on, 14-18 during cruise, and 8-14 during acceleration. With the exception of the XE-274 in the 305 the only time I ever saw 0 vacuum was when the pedal is on the floor and even then their is about 1 in/hg vacuum when the RPMs come up or more.
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Old 07-15-2005, 03:34 PM   #11
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I made sure i had it hooked up to the manifold vacuum, i hooked it up to the timed vacuum and i only registered when i reved the engine. i am hooking it up to the carb on the port for the distributor
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Old 07-15-2005, 04:52 PM   #12
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Fast355's results are what they're supposed to be, but when I checked mine, it was closer to mrt's, around 14" at idle, but went *up* to 21" or so when revved.
This is testing on a manifold hole (I hope...), at 4000' above sea level... I think it's because at a 550RPM idle, the vacuum isn't high, but when revved to 1krpm it will be... This is testing when leaning over the engine with a gauge, not a pod in the car, so I can't say under load or anything. (I did notice other holes gave a 0 reading until revved, then it'd pickup, but still be low, 7", and I assume that's my ported... then again, q-jets have some weird holes...)
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Old 06-27-2013, 09:52 AM   #13
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Re: Tuning carb with vacuum gauge

On a carter carb do you use the PVC outlet or the manifold vacuum port on the carb?
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Old 06-27-2013, 02:59 PM   #14
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Re: Tuning carb with vacuum gauge

You need to have a good baseline with other adjustments before you adjust the idle mixture.

1. Hook up vacuum gauge to manifold vacuum.

2. Set the ignition timing.

3. Set the idle speed (in gear if it's an automatic).

4. Turn the mixture screws out to get the highest vacuum reading and then stop - you're done. If you don't see any increase when you're turning them out, turn the screws the other direction until the reading falls and then turn them back out to get the highest reading.

5. Re-adjust the idle speed if needed.

The idle mixture screws will have no effect on the mixture when the engine is above idle speed.
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Old 06-28-2013, 01:03 AM   #15
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Re: Tuning carb with vacuum gauge

I set the idle screws to the highest vacum, about 23 is what I got. but I went by the PVC port, should I go by the manifold vacuum port on the carb?
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Old 06-28-2013, 08:31 AM   #16
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Re: Tuning carb with vacuum gauge

Quote:
Originally Posted by evilemokid94 View Post
I set the idle screws to the highest vacum, about 23 is what I got. but I went by the PVC port, should I go by the manifold vacuum port on the carb?
It doesn't matter, just not the timed port.
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Old 07-02-2013, 12:54 PM   #17
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Re: Tuning carb with vacuum gauge

you can get the curb idle pretty close with the vacuum gauge but from there on it's useless without a wideband air fuel meter
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Old 07-02-2013, 04:05 PM   #18
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Re: Tuning carb with vacuum gauge

My setup gets 13 inches of vacuum at a 800 RPM idle with 18 degrees base timing according to my vacuum gauge.

It runs great, pulls hard. All the plugs look fine. I figure the gauge isnt calibrated correctly and left it alone. I probably dropped it at some point or something. Or maybe it's just a cheap parts store vacuum gauge. (It's a "diagnostic" style gauge)

Anyway, I just tweaked for the most vacuum and left it alone.
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Old 07-02-2013, 04:42 PM   #19
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Re: Tuning carb with vacuum gauge

It works fine for the idle circuit. Last time I did it the idle circuit was exactly where it needed to be once on the wideband dyno.

You'll want to start with all the screws the same distance normally 1.5 to 2.5 turns out or so. Then you'll work around the carb slowly adjusting the screws all in the same direction. You'll probably want to go 1/2 turns on each then recheck vacuum to see if it dropped or increased. Eventually you'll be adjusting by 1/4 turns then 1/8 turns searching for your highest vacuum. At that point it should pretty dead on. In my exprience it's pretty hard to find anything smaller than 1/8 turns.
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Old 07-02-2013, 04:42 PM
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