Hi I am having the same problem, or so it would seem
have taken the carburetor apart at least a dozen times.
With the air filter off, the motor is sweet running and accelerates reasonably well, as soon as I put the air filter on (brand new filter, free flowing air) it wont rev, gets about halfway through the rev range and bogs down, over fueling, I suspect. Spark plug is a little black.
have checked all jets, all clear.
checked the diaphragm, no leaks, works perfect without the air filter, I can see it move up as you accelerate the motor. Spring on diaphragm seems good too.
airscrew perfect, idle is smooth
choke seems to be perfect, not loose and returns fully.
Motor starts very easily.
Please help, have no idea what to do next?