I feel a notchy vibration when decelerating, though the footpegs, most notably when i have my weight on the footpegs. Several cycles per second. When cruising at 80-100 KPH and in 7,000-8,000 RPM but only when i let off the gas. When i pull in the clutch it goes away. Chain and sprockets are new. Chain slack is adjusted properly. Though i did have it too tight at first. Rear wheel bearings are new. Tranny oil is up to level and fresh. When i'm on the gas this vibration goes away completely. 1991 MC21. 8,000 KM. Any ideas?
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MC21R9N track
MC18R2J track
CBR954
CR250

I would double check the engine mounting bolts to make sure they are tight . Worth a try

I would have said check the chain for tight spots as you rotate the rear wheel.
But with new chain and sprockets, I doubt that's your problem.