Assuming one adjusts the chain as per factory spec, there will be a small amount of "play" somewhere. During acceleration since the jackshaft is accelerating in RPM, the chain will be very tight on the backside (between about 9 o'clock on the bottom gear and about 9 o'clock on the top gear). The "play" has to be somewhere but it's not on the backside during acceleration. So, either it's gone because the chain is spinning and it technically wants to form a circle, or the play is on the front side somewhere. Enter the spring loaded tensioner roller. I think its there to be able to move and absorb "play" when needed. During hard braking (and to a lesser extent during coasting or decelerating), the "play" moves to the back side and the front side gets really tight increasing load against the tensioner roller and spring. If the chain is too loose, IMO its more of a problem during hard braking than during acceleration because the sudden shift of the "play" from the front side to the back side which makes the tensioner slam against the adjusting bolt. At least during acceleration, even if the chain is too loose, the spring loaded tensioner can somewhat take up the slack enough to keep it from skipping or derailing. Too loose is not good. But, it seems too tight causes bushing failures. So, giving Yamaha the benefit of the doubt, they came up with 1-1/2 turns out from finger tight. It seems at that setting there's a better chance of acceptable bushing life. We need a scientific study under controlled conditions to definitively understand what in the world is going on in the that chain case (sometimes) to make it destroy that bushing. Some guys report long life with no virtually no wear, others with almost identical setups and sleds have very short life and failures. Frustrating, maddening, infuriating, etc, etc.
