You have a fuel problem. It's either the fuel itself or a delivery issue.

Have you tried a fresh can of fuel?

Is the pressure nipple on the muffler partially blocked? Pull the muffler, blow into one end and cap the other. You should have a nice steady stream of air coming from the pressure nipple. Try cleaning or drilling it out with the appropriate drill bit.

Put a new piece of fuel line on the carb and blow through it. Work the throttle up and down. Does the air flow proportionally to the carb setting?

Is the governor working correctly? Have you tried to run the motor without it since the trouble started?