I have had symptoms very much like this and turned out the gas went bad in the tank. Any chance your fuel is old? The new fuels go bad much faster than the fuels of yesteryear(the ethanol maybe). Try dribbling a sip of fresh gas from a can down the carb, put a breather back on to protect against a backfire, and roll it over. I know it doesn't make sense but the fresh fuel down the carb trick has helped me rule some things out or enlightened me many times. JUST BE SURE TO DO IT SAFELY. I knew a guy that was doing this while his son cranked the engine over, it backfired and burned him badly. In extreme cases of old/bad fuel the plugs become fouled and must be replaced. The bad gas usually is accompanied by a very fuming smell when if/when it starts and runs in a seemingly lean condition. Also if you do rule out bad gas or know this to not be possible, take a look at the throttle shaft bushings and if all else fails pull the lid and look for debris in the carb.
...Jason
...Jason

