I may be wrong, let's run the numbers. I have not tried a 36T sprocket on the Point Beach bike I have, so the experience of those who have; will of course over-ride my thoughts on this.
I assume you meant to say "if the engine can't rev up high enough to get "on the power curve" he would suffer a lower top speed than with the 44T. Yes that's true, but looking at the numbers, I can't believe these engines have such a narrow power band!
The reason some motorcycles go faster in fourth than fifth is wind resistance. At 100+ mph wind resistance is huge.
Assuming a bicycle with a 26" diameter rear tire is used, the numbers work out as follows.
Pi X D = circumference, or a circle laid flat.
One revolution of a 26" Dia. wheel will make the bike travel 81.681 inches or 6.806 feet.
There are 5,280 feet in a mile. 5,280/6.806 = wheel revs per mile.
776 wheel revs per mile.
32 miles per hour will correlate to 24,832 wheel revs per hour.
24,832/60 = wheel revs per minute @ 32 miles per hour. 414 per minute.
414 x 4.4 for a 44T sprocket = 1821 jackshaft RPM
414 x 3.6 for a 36T sprocket = 1490 jackshaft RPM
Engine RPM through the 4:1 ratio clutch gearing...I'm not at the shop so I can't accurately count the gear teeth. from photos I'm sure 4:1 is close. It looks like 80T on the clutch gear and 20T at the crank from photos.
44T = 7,284 engine RPM
36T = 5,960 engine RPM
A difference of 1,300 rpm well into the powerband of this engine.
Working back through the numbers 21 MPH and 36T would require an engine RPM of 3,911. Seems like enough RPM for the engine to be making power?
Georgeknight: Have you tried a fresh NGK plug?
I don't know, the numbers seem to suggest the problem lies elsewhere.