Go one size leaner on the main (stock is a 70, go to 67.5) and tune the pilot first. The PW50 is rich from the factory, and tends to be abnormally affected by the screw even at the top end.
You can, but you need a dial indicator to measure the stroke of the pump.
Basically, you need to remove the cover, mount the indicator in a very stable location in line with the plunger, and adjust the stroke with a wrench while the engine is running. The exhaust will probably be in the way as well. You can get a Clymer manual for the bike for around $30 that tells you how to do that, and a lot more.
My son's 200 PW50 starts easily, won't run crisply or clear out at full throttle, and the plug is fouled. It is completely stock as far as I can tell (but the restricting exhaust washer has been removed).
Any suggestions/ideas?
Thanks,
Rob