Carburetor 101, help me out please... -
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post #1 of 3 Old 02-19-2003, 06:35 PM Thread Starter
4x4 XJ
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Carburetor 101, help me out please...

Carburetor 101, help me out please...

I was into FI and never had a carb before. About to get a 89 YJ Islander 258 4.2 carbed. Stock carbs. How exactly does the carbs work, total newbie to carbs. In terms of injection, pumps, fuel filters, etc...what's a good upgrade to the stock carbs aside from setting up a FI system, might not be willing to do $1500 in FI material so what carbs are good as far as tuning capabilities and fuel mixture? What's all this "choke" and "cold start" and this and that...please give me the whole enchilada on Carbs 101 please, anyone that can help me out, everyone that can help me out. Thanks a lot all.


1997 ZJ Limited 4.0 H.O.
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post #2 of 3 Old 02-19-2003, 08:56 PM
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A carburetor is simply the crudest, simplest device for mixing air and fuel together. Unlike your FI which squirts fuel into the individual cylinder, a carb mixes it together then the intake manifold divides the mixture up into the different cylinders.

That said, there are really only 3 basic things to consider with a carb--mixture (rich or lean), idle speed, and fast idle/choke. Of course, if your engine isn't stock, you can get a lot more precise by changing jets, etc, but you generally won't have to get into that if your engine is stock and the carb is matched according to size.

The mixture is simply the ratio of air-to-fuel. Too much fuel and it's rich=soot in the exhaust, bad smell, fouls out spark plugs. Too little fuel and it's lean=too hot, inadequate power, flat spots, etc. You either have one (weber) or two (nearly every other brand of carb) mixture screws and the basic idea is to run the carb as lean as possible without sacrificing power.

Idle speed is pretty self-explanatory, you just adjust the screw to make the engine idle at whatever speed you want.

Fast idle/choke is for when the engine is cold. Cold engines don't want to run, as a rule, so the carburetor richens the mixture (via a choke, which restricts the amount of air) and artificially speeds up the idle speed (which again, you set to your preference).

That's the basics of it, and since I've already written a book here, I'll stop. If you want to get into the specifics of tuning a carb and/or examples of proper idle speeds, fast-idle speeds, etc., just say so and we can go further in-depth--

Currently an '86 CJ-7 and a '77 CJ-7 Golden Eagle---#4 and 5 following a '01 Sahara, '90 Sahara, and '85 CJ-7....
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post #3 of 3 Old 02-20-2003, 06:28 AM Thread Starter
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Thanks for the basics...I would like to know more but I am going for the 91+ YJ with the 4.0 FI I know most about and the AX15 that I trust, thanks again for the info though...

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