'06 YZ 450 vibration on decel

I recently bought a nice used '06 from the second owner who had the bike for about 6 months... the bike looks to have low hours (hardly any wear anywhere), all stock except FMF slip on and recently installed 6.15 flywheel. The guy I bought from was meticulous with his bikes (this was his "extra" bike - he has a bad ass KTM 350 as his main ride and a couple bikes for his kids) and he recently rebuilt the top end with new cylinder, piston, rings, timing chain, etc. He put an hour meter on after the rebuild which read just over 4 hours. He said the valve train was still in great shape, valves never out of spec, etc, so he left them as is. He runs full synthetic in all his bikes, the bike starts first or second kick every time, hot or cold, runs good, crisp response, etc.

Here's my question: I took it out for the first time last weekend and noticed a vibration on deceleration. I never noticed this when I test rode at his house. Could this be a bad crank or main bearing? timing chain or tensioner? the heavier flywheel?? or is this normal? I've owned Yamaha four strokes in the past (a couple 426s and a 250F) and have never noticed this type of vibration before, even on my CRF450. It wasn't arm numbing, but just enough to be noticeable. I put about 6 hours on it last weekend in the desert and it ran fine. I checked the engine mounts and all other nuts/bolts and every thing was good. I'm worried now that I bought a lemon :bonk:

Does anyone have any insight on what this could be?? help! :smirk:

Check the motor mounts and swing arm pivot bolt.

The motor mounts checked out fine, as did all other nuts/bolts... but I didn't think to check the swing arm pivot.

forks need a rebuild?

Vibration on decel (assuming that all the motor mount bolts are torqued) is usually an indication of a bad rod bearing or wrist pin. Is the vibration accompanied by any change in engine sounds or other subtle sound changes?

The motor mounts are torqued, no change in engine sound, and the wrist pin was replaced with the top end. He had it rebuilt by a guy (who is a friend of his) who is a tech at a local Yamaha/Kawasaki/KTM shop... he did the rebuild at home. Could the heavier flywheel have anything to do with it? what about the new timing chain? perhaps something wasn't installed correctly when the top end was rebuilt?

Could the heavier flywheel have anything to do with it?
If the flywheel were incorrectly installed or out of balance, it should shake all the time, not just on decel.
what about the new timing chain?
perhaps something wasn't installed correctly when the top end was rebuilt?
Maybe. Are you sure it's unusual/abnormal? Does it have an aftermarket skid plate?

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