Just a guess, but it sure sounds like a fuel problem.
I'll assume the float chamber is about 1/2 full of fuel when you drop if off of the carb? If there is much less than that, I would suspect the float or float valve. Also, if the tank valve has a filter or there is one between the tank and the carb, it might be plugged up.
Disconnect the hose at the carb and turn on the fuel valve. It should have a healthy flow. If it just dribbles, that's the problem area.
If the float chamber is about 1/2 full and the fuel line is delivering a good flow of fuel, one of the jets in the carb is probably plugged.