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To prevent this sort of difficulty in the future, learning how to troubleshoot the problem would be a good idea, IMHO. What you have done is replace good working parts in an attempt to fix an undiagnosed problem. I’ve done that and I’m sure others around here have done it too.
You have spent probably $200 plus hours of time & frustration w/o having the slightest clue as to what is causing the latest problem. Worse yet, by replacing that carb, you have probably introduced more problems. The new carb you bought could easily be the cause of it only running on choke. Instead of buying a new carb, checking the fuel screens (tip # 45) would have been a better alternative.
Pulling out the choke gives you a richer mixture. So, you could have either a fuel or spark problem. A carb out of adjustment will also cause this problem; set the main jet at 1-1/2 turns out & leave it alone. A dirty carb can cause the problem as well; start by cleaning the fuel screens. (see tips 45 & 56) Dirty/old gas, or water in the gas can cause this problem. Restricted fuel flow can cause this problem; remove the bolt at the bottom of the carb & check for flow. The flow should fill a pint jar in under 2 minutes. On my 1950 frontmount, needing choke to run is the first sign that the points need adjusting/replacing. A vacuum leak can do the same thing. Get a hand propane torch and carefully spray it (unlit of course) around the intake manifold at the carb to manifold interface & the manifold to block gasket of the tractor while it is running. Or, you can use carb cleaner or WD40. If it speeds up, you found the leak. Sometimes (rarely) manifolds crack or get rust holes. Lastly, when all else fails, you could have an air leak around the carb metering section. The likely suspects are leaky throttle shaft seals and/or a worn throttle shaft. Or, just a real dirty carb w/ clogged passages. Bottom line: Probably 90% of "needs choke to run" problems are fuel related, but do not rule out ignition problems, contrary to what others may tell you. I’ve had that very same problem before w/ a rebuilt carb, fresh fuel & good flow & a tight manifold..... .and it was a spark problem. But, that was unusual. Just check the likely fuel problems first. And do not buy a new part for the tractor until you can answer this question: how do I know the part on the tractor is defective?
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