Just the fact that the motor performs so much better when you spray starting fluid into it tells me that there's something not right with the carb. I would look into:
float needle seat
plugged passageway (take a piece of real thin wire and run it through those small areas)
I don't suppose that you have access to another carb that works? That would narrow down the problem.
Keep in mind also that when you spray starting fluid in there that it's quite a bit more combustible than plain gasoline. Have you tried pouring a little plain gas in there? If it speeds up when you do that then it's definitely a carb problem.