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When the engine revs are high it runs well, the air adjuster screw is set at recommended setting (1.5 turns) and at low rpms the engine sputters out, but isn't flooding. I've tried a replacement carb and it runs fine, so the problem is within the carb.

I've tried opening the air adjuster screw, no effect. I replaced the upper half of the carb with part of another, and the problem is definitely in the lower part of the carb (the part that bolts on the intake manifold, where the air mixture screw and butterfly valve is.)

Thanks for any help / suggestions.

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I'll assume first that you've tried spraying carb cleaner into it since you've had it out, so your question is more about what other adjustments there are, specifically relating to the idle.

Beyond the Mixture screw you mentioned, there is usually 'Pilot Jet(s)' that allow some fuel regardless of RPM (as opposed to the 'Main Jet(s)'), and if they get gummed up that could cause your issue. If they're clean (and identical between the carbs), the next thing I would check is the float bowl height. Here's a good intro:

http://www.youtube.com/watch?v=cjIjExNumJA

Not only can they 'leak' and allow some gasoline into the bowl, the springs/tabs can weaken and put them in an unequal/improper place. This will sometimes even out in the higher RPMs.

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  • There's also the throttle stop that sets the rest throttle postion.
    – Ehryk
    Jun 2, 2012 at 0:52
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Ehryk has a lot of good suggestions. If it's not flooding out (rich) then it's got to be running lean. A common problem with older carbs is air leaking in around the throttle shaft. That's the shaft to which the butterfly valves are attached there may or may not be replaceable bushings. If there are no bushings, or bushing kits then it's time for a new carb.

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