Been working on a new to me TO 30. Got the axle and pto leaks taken care of and moved on to what I thought was carburetor issues. When I first bought it the tractor would run fine for 5 to 30 minutes then start running rough. I would mess with the the main jet and it would run for awhile again and then run rough. I thought there must be junk getting into the carb from the tank. I flushed out the tank, put a new sediment bowl on, flushed out the fuel line and cleaned out the carb. Now it only runs with the choke about three fourths out but with the choke in the right spot it runs good. It has a zenith carb on it. Any ideas?