Your carburetor jet is clogged, do you have a single cylinder engine or twin? Carb jets are tiny and can be clogged by very small debris.
SINGLE- Drop the carb bowl and clean it. Going from the bottom up in the center of the float, you'll locate the jet. Clean it with a toothpick or piece of wire in the center by pushing either up into the hole. You may also want to spray some carb cleaner to wash any small particles away afterwards.
TWIN- Remove the circled plug (attached photo) to locate the jet and perform same procedure as above.
