Top the tank right to the bottom of the filler neck. If the engine now runs, you either still have a problem with the (pump) diaphragm you just replaced, or you have a blocked pick-up pipe. Remove the carb from the tank, and check that the long pipe is not blocked (blow through it. If you can blow through it without gomg red in the face, it's OK).
If the engine does NOT run with a full tank, the problem is probably in the short pick-up. Test as above.
If all that checks out, the carb wil have to be properly stripped and cleaned.
Phil Saunders, March 2009
