carb burning too rich

andrewhove

New Member
Joined
Jan 24, 2008
Location
Durham
have a 77 cj with a 304 in it with the stock carb. It's burning to rich and leaves really rich exhaust wherever it goes. how could I make it run more lean?

thanks
 
Im assuming you have tried adjusting the carb?

or are you asking for instructions on how to do that?
 
If it's a two barrel, it might be an "MC2100"
Very easy to rebuild. Should be able to adjust idle mixture with two screws sticking out of the lower front of the carb. Screw them all the way in (NOT TOO TIGHT OR YOU'LL RUIN THE SEATS). from there you turn them both out 3 turns each.

Once to this point, you move them equally in or out a 1/4 turn at a time. If it's idling too rich, you'll smell it at the exhaust outlet. The idle mixture screws shouldn't be turned out any less than 2-1/4 turns. so from 3 turns out you turn them in 3/4 of a turn & it still smells rich then the jets need to be smaller.

Go find an autolite of motorcraft carb at the junkyard & raid the jets out of there. Check yours first to see what size they are.
If the stock ones are say #48F & it's running a little rich, then you need #47F or #46F

One more thing, when you rebuild the carb you'll get a 7.5HG power valve in the kit.

To find out if that's the correct one for you, follow the instructions in this vid;
http://www.youtube.com/watch?v=mE_xTUXQJQ8

Mine needed & actually ran better with an 8.5HG power valve. MC2100's use a holley type valve that are available at most speed shops.
 
The two idle mixture screws should be under the fuel bowl facing the front of the engine near where the carb bolts to the intake. They can be kind of hard to see, but they are usually (on a motorcraft/autolite 2100/2150 carb) a straight screw head with a spring behind the head. If the carb was an emissions carb, then it may have tamper resistant caps over the adjustment screws. A rebuild kit will explain how to remove the tamper plugs if that is the case.

To adjust usually you turn the screws in until lightly seated like stated above, then back them both out the 2-3 turns. Then you would adjust evenly (1/4 turns each) until you get the highest rpm reading or highest vacuum gauge reading. After that you set the idle speed by adjusting the screw/solenoid on the throttle lever to the desired idle speed. If it still runs rich then you may need to rejet the carb like the above post said.
 
Those screws only adjust the Idle mixture. Once it s off idle(any throttle) it's not on the idle circuit anymore. Adjust that all you want, but from your description you said it's running rich everywhere I'm guessing you mean under throttle.

I'd honestly suggest a rebuild. that way you know your stuff is good and sealed up , and you can learn how it works. the MC2100's are stupid simple to work on so I say have a go at it.
 
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