There's virtually no way to do a "carburetor rebuild in a can" successfully. I agree with the idea that you've got a stuck power piston, but once the engine fires the first time and runs for a minute or so, there shouldn't be a need for much, if any choke. It sounds like it's time to dismantle and thoroughly clean the carburetor, rebuild it, and then DRIVE it more often. Short trips and "warm-up" runs are death on carburetors. Make 'em earn their keep with some highway-speed trips frequently.
Jerry
