When checking the diaphragm replace the main needles and needle jets.
You can't check them as the minutest wear on them will have a major effect.
You will know it is the needles and needle jets if the bike refuses to rev over 4000RPM but has lots of power in the lower revs and still runs evenly balanced. Damaged diaphragms will make the machine run rough and unbalanced as you start to accelerate and you will experience a lack of power through the rev range although it will get gradually worse as the revs pick up. Worn needle jets and main needles will almost have a certain cut of when you get the feeling that the bike refuses to continue accelerating.

The main needle is attached at the bottom of the piston and the needle jet will be held in place by the venturi directly above the main jet. The main needle fit through the needle jet. To get to the needle jet you have to first remove the main jet and venturi and the rest will drop out, or if heavily gunked up, will need a gently push with the tip of a screwdriver.
When buying the needle jet make sure you get the right size. Remove the ones on your carb and you will find the size stamped on the side 2.64 or 2.68 yours will probably be 2.64 If not, it could also be that they have been replaced by the wrong ones before.
Give me your bike model and year and I can check on the electronic parts catalogue what was supposed to be on your bike.