If you have verified that the choke is good. That there are no problems with the high idle. And that the curb idle is set properly. Then next I would:
Rebuild the carb. Break it down entirely. Soak everything in carb cleaner/acetone overnight. Being sure to remove any plastic parts of course. Blow carb cleaner through it (pressurized can type). Then blow compressed air through all the passages. Re-assemble. And follow
AJ's guide to Transfer Port Synchronization to properly set it up for idle.