Have a 1940 Allis B here with similar problems. Mine starts and idles fine. I'm running the Zenith carb with the needle screwed all the way in to the seat. If the needle is out at all, it needs choke to idle nice.
I can idle up 3-4 notches before it starts to stumble, but after that need choke for it to run well. It does pull really well with a little choke. I've had the carb apart 3-4 times 2 years ago and replaced most all of the parts including throttle shaft and choke butterfly shaft with a kit from Steiners.
This year saw an ad on Craigslist from a local guy that offers ultrasonic carb cleaning. Took the carb to him, and he looked it over. He thought a dunk in his ultrasonic cleaner would be a waste of time because it had all new jets and everything is clean inside.
I've been trying to figure out what might be wrong. I know that Steve says main jet has to be plugged, but don't think that is the case here. Have seen a couple of guys online talking about internal air leaks in older Zenith carbs. The ones I saw were aluminum-body carbs that warped. Symptoms they describe seem very similar to what I'm experiencing here. The suggest sanding the body where the split is to remove the air gap. Have tried that, but being cast iron don't think that was the problem necessarily.
The venturi doesn't seem to seat particularly well. . . . Wondering if there could be some other place where there is an internal air leak?
Tractor runs OK, and I can get work done with it--just getting tired of continually adjusting choke whenever I throttle up or down. Thinking about buying a new carb online, but wondering if I'll just have other problems down the line as likely Chinese and may have other flaws? Old, used carbs on ebay don't look even as good as what I'm running, so hard to believe they would give me anything but more problems.
Not much help, right?
Jim
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