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post #1 of (permalink) Old 12-18-2000, 10:19 PM
MarkF
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Carb tuning for dummies - need info

I ordered a book the TR recommended to help set up the 4 barrel Holley on my Chevy 350.

In the meantime I was wondering if there were any sources on the web that go through the basics of tuning/setting up a carb. Right now my carb is running too rich and I haven't a clue.

Thanks
Mark F

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post #2 of (permalink) Old 12-19-2000, 09:30 AM
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Re: Carb tuning for dummies - need info

You can tune a carb pretty easily using a vacuum guage.
Hook the guage to a source of Manifold vacuum. Then turn the air/mixture screws down 1/4 turn each until the vacuum just starts to drop. You should be right in the ballpark at this point. See how she runs and you can fine tune it by using small adjustments now....Remember to turn each of the screws exactly the same amount. It may take a couple of tries to get it perfect. Hope this helps.

Tommy B. in AZ -
More Guts than Brains.
post #3 of (permalink) Old 12-19-2000, 10:09 AM
MarkF
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Re: Carb tuning for dummies - need info

OK, I have a vacuum gauge, do I start where the screws are now, even though it currently running too rich, or do I turn them both all the way out, hook up the gauge, and start turning them in?

Thanks
Mark F

1986 CJ7, Chevy 350, D44/D30, more to come shortly
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post #4 of (permalink) Old 12-19-2000, 11:58 AM
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Re: Carb tuning for dummies - need info

If I were you, I would turn both the screws all the way IN just to where they stop. BE VERY GENTLE.. Don't overtighten the screws!! Mark the screw and carb body with whiteout or something. This will be your reference point so you can tell how much to turn the screws. Then back both screws out maybe 1 1/2 or 2 full turns. Hook the vacuum guage to manifold vacuum, not ported vacuum. Verify all other vacuum lines are connected. Start the Jeep and let it fully warm up. (You may need to back out the screws a little more to keep the engine running at idle). The key is to keep them turned the exact same amount. As you turn the screws out, (1/4 turn at a time for an equal amount), watch and see where the vacuum reaches its highest point. You can tell the highest point because the vacuum would not increase any more if you kept turning the screws. Right at the point where the vacuum is highest, and if you were to turn the screws in at all, the vacuum would start to drop is where your carb should be real close to proper adjustment. You can fine tune it using smaller adjustments, but you will be real close with this method. Remember both screws need to be turned the exact same amount. Good luck and let us know how it works out...

Tommy B. in AZ -
More Guts than Brains.
post #5 of (permalink) Old 12-19-2000, 12:39 PM
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Re: Carb tuning for dummies - need info

Is the carb running rich all the time or just at idle? The procedure with the vacuum gauge works great for adjusting the idle mixture, but if it is rich all the time don't expect this to fix that problem. Once you start to give the carb a little throttle the idle circuit is no longer in use and the mixture screws don't do a lot. This is where the tuning book will come in handy.

'75 CJ5,258 w/Howell EFI,T18a,4.27's,33's,On-board air,Warn 8274
post #6 of (permalink) Old 12-19-2000, 12:40 PM
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Re: Carb tuning for dummies - need info

Is the carb running rich all the time or just at idle? The procedure with the vacuum gauge works great for adjusting the idle mixture, but if it is rich all the time don't expect this to fix that problem. Once you start to give the carb a little throttle the idle circuit is no longer in use and the mixture screws don't do a lot. This is where the tuning book will come in handy to figure out the proper jetting to avoid running rich.

'75 CJ5,258 w/Howell EFI,T18a,4.27's,33's,On-board air,Warn 8274
post #7 of (permalink) Old 12-19-2000, 12:43 PM
MarkF
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Re: Carb tuning for dummies - need info

It is running rich all the time. I ordered the book, the problem is that I have to pass inspection and I want to eliminate all of the obvious problems hoping that they'll forget to check the not so obvious things like cats and EGR.

Thanks
Mark F

1986 CJ7, Chevy 350, D44/D30, more to come shortly
post #8 of (permalink) Old 12-19-2000, 01:13 PM
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Re: Carb tuning for dummies - need info

That carb runs rich unless choked back with the mixture screws. Attach a tach and:

1. Turn the right mixture screw CW untill you see a 100rpm drop.
2. Turn the right mixture screw 1/4 turn CCW.
3. Repeat step 1 and 2 for the left mixture screw.
4. Repeat steps 1 -3 untill the auto starts to die...

At this point turn the mixture screws 1/4 turn CCW.

You've now leaned the carb, and kept it running... it should pass emissions.

Happy Holidays and Safe Jeepin'

Larry
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post #9 of (permalink) Old 12-19-2000, 01:17 PM
MarkF
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Re: Carb tuning for dummies - need info

Thanks Larry, I'll try it as soon as I can. I assume I do this on a warm engine, correct?

I have a tach in the cab, which will only help if I get a second someone else to watch.

I also have an analog multi tester (Sunco, I think), I think I recall seeing a tach setting on that. If so where would I hook that up to?

Thanks again,
Mark F

1986 CJ7, Chevy 350, D44/D30, more to come shortly
post #10 of (permalink) Old 12-19-2000, 02:09 PM
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Re: Carb tuning for dummies - need info

You are correct sir, perform the adjustments on a warm engine. After I did this process a couple of times.. well I don't use the Tac anymore, I just tune by ear, listening for the RPM drop. You'll soon be able to do the same thing... and then it's a one man job.

Happy Holidays and Safe Jeepin'

Larry
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