Nathan..

Check all this stuff above..

The tappet clearance should be 0.1mm. using a 10 feeler gauge yu should be able to pull it through the gap with some friction when the piston is at tdc.

If you have not adjusted these since new they almost certainly need checking.

Some of the later checkvalves (last few years) have been known to cause probs where the plastic disc swells depending on fuel being used... make sure it can move freely in the barrel.. blow one end gently then the other... you should feel or hear the disc move.

As someone said, if you remove the diaphram / brass screw take care, and note which way round things go back. The screw should be reset so it is flush with the regulator body.
Do this clean up job in one hit and do not allow the diaphram to sit outside for long... you dont want it to dry out.
DO NOT put any oil in the regulator or anywhere in the system... it will block everything up and it will take a tank of fuel and mucking about to clear it all out.

The idle screw should be 1 turn out from fully closed, and main needle 1 1/4 to 1/12 turns out depending on how much Nitro is in use...

y63s are gems of motors.....