If it ran good before, but now has less low end power, I'd think there's a mechanical problem.
When I bought my XC 200 used, it ran poorly at low rpm's, sort of like a high strung 125 mx'er. Air screw made no difference. I went up one size on the pilot and it ran much better. Now I could fine tune the idle with the air screw. Bottom end power was very good, for a 200.
Not too many hours of riding time later, the engine seized up after a hard hill climb requiring high rpm's. long story made short(er). the left side crank bearing let go, thankfully nothing else was damaged.
I believe the crank bearing was going bad when I bought the bike. The crank bearing was allowing the crank to move a little and allowing the left side crank seal to leak, causing the lean condition that I thought i had fixed by going up (richer) on the pilot jet.
Not saying that's your problem but it is something to consider, imo.
RB
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