Unless the air filter is dirty I would suspect dirty carb. leaning it out. Try spraying carb cleaner down the carb venturi first while it running with short bursts while triggering the throttle with the engine at a high rpm.

If that don't work you may have to disassemble the carb and clean out the passages. At last resort if you can adjust the carb. you might try to richen the mixture up alittle, usually only takes 1/8" turn out or less.