I'm not exactly sure what your problem is, it sounds like you're saying you are not getting top end revs??
I have had an 021 for at least 10 years, same one. I have had to replace the rubber fuel line twice as it perished and had micro cracks that would suck air when throttle was applied, so no top end revs. Check that first.
Also I had to retune my carb, set everything to factory specs, as marked on the saw casing, warm it up then set the idle so its about 2000 rpm, a lot of people set the idle too low, it needs to sound fast. Just make sure the chain does not spin with the chainbrake off.
Oops, my 021 is in the van just now, so I can't recall the other setting screws.
Check back tomorrow
BTW Its a great little saw, once I got it set right, no problems! Will cut forever.