I think atomark has probably hit the nail on the head. sounds like trash/water in the main jet. Do the power flush thing (hand over the carb while running at high rpm) If it clears up invest in some fuel treatment to mix with the water that may be in your tank. Dont freek out when the carb. sucks you hand like a vacuum cleaner on you cheek.

I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"