If it doesn't run with the choke off (open) then it sounds like an air leak somewhere. Start with the intake manifold/carburetor area and go from there. Many of us use a spray carb cleaner to check for air (vacuum) leaks. If the engine RPMs increase with the introduction of the carb cleaner that is where you want to look for, bad gaskets, defective seals etc. Excess air entering the engine will produce the condition you're describing. Get back to us with your findings.
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