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Carter idle mixture screw adjustment

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My 1954 Dodge truck has a Carter single barrel carburetor.  Turning the idle mixture screw all the way in does not seem to "lean" the mixture out. In fact, the engine seems to idle best with the screw all the way in.

This probably indicates that the carb needs rebuilding. Does it indicate anything else specifically ?  Where would the carb be drawing gas from with the needle all the way in ?


Edited by WPVT (see edit history)
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Check Timing and idle speed.


The idle jet provides a mixture of air and fuel out the needle. Right above the hole the mixture screw sticks through is a "transfer slot" or "transfer port". it is fed from the same air/fuel that the idle jet needle is. Normally it would be above the throttle plate and not feed any air/fuel. If, due to tuning issues, like retarded timing, or a miss, or a vacuum leak, or whatever, the throttle is open further than it normally would be at idle, then part of the transfer port will be below the throttle plate and feeding air/fuel. You would be left with little to no control by the mixture screw.


The throttle shaft could be worn out, causing the throttle plate to hang up on the bore. Idle would be too high, but otherwise the same situation as in the paragraph above.


The float could be set too high, or sunk, or have a bad needle and seat causing the fuel level to be too high. Assuming that the throttle was found NOT to be too far open, If AND ONLY IF the engine has no tendency to backfire, you could do the following. Put on some safety glasses, get a bright flashlight, take the air cleaner off, and stare down the bore at idle. You should NOT see any fuel feeding anywhere. It should all be coming out the mixture screw, under the throttle plate, where you cant see it. If anything is coming out of the main discharge tube, the float is "running over", meaning the bowl is too full, probably for one of the reasons above. It works like a toilet. It fills to a specific level, then the float and valve shut it off.


Check the tuneup issues first, and be sure the throttle is mostly shut. Its easier. You may need carburetor work.

Edited by Bloo (see edit history)
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Thank you. That's very clear and informative advice. I know exactly what you are talking about, and I'll take a look today. 

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