My '80's carbs don't have diaphragms , the "90's do , maybe they use the older carbs down there? They simply rely on the piston of the slide to be close fitting to the cap bore, not the best design .
I think he is simply out of power for the too high gearing. I have several XT's and they are not powerhouses that can pull any gearing wanted. I run 15/40 in 2 and that is about all they can handle while still being able to pull fully(just barely with a small headwind @ 65mph). At most I could go down to 38 rear but headwinds would limit topend.
The advance will probably make a huge difference in the upper rpm's.
For more power a high comp piston/mild porting and big headpipe with can will get quite a bit more oomph . A better set of carbs will also greatly improve performance with some tank range loss.
TURBO !!!!
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