(1) In-line fuel filter, then clean the carb. Tiny amounts of rust in the fuel tank will move through the straner and casue problems. I didn't belive this until someone convinced me - and since I've added one to a B I own I've had no more problem.
(2) Check your intake manifold for leaks or holes. This usually means it runs too lean, but it casn make a huge difference. Some leaks or holes only show after the manifild warms up to operating temp.
(3) Make sure your carb float doesn't have a hole in it - shake it to make sure there's no gas inside (you already have done this I'm sure).
(4) Make sure your timing is set correctly. Incorrect timing can make a tractor do wierd things.
(5) The governor could be worn, but it should still run ok sitting still (I think).
(6) Use a single strand of wire from a 16 gauge stranded electrical wire to make sure all the ports are open in the carb.
(7) Make sure your distributor cap isn't cracked - I'm assuming you put a new one on.
That's all I can come up with for now. Burned valves or incorrectly adjusted valves can do some interesting things like this too.