ionbeam
2 FUN
To add on a tiny amount to what Fred Sez --
As the screw and lock nut rocker arm type valve train wears and starts to clatter the valve openings (lift) become smaller which improves low rpm port velocity and you will start to notice better low rpm power with an accompanying drop off in high rpm power. This is pretty much a fail-safe valve system, the valve lift gets smaller and performance will diminish until it becomes unacceptable, with no harm.
As shim/bucket valve trains wear valve lift increases resulting in lower port velocity at low rpm causing a drop in power and the higher lift increases breathing at high rpm allowing higher power. At least right up until the valves don't fully seat, then as Fred sez, the valve flute and/or the cylinder head valve seats start to silently burn. Check those valves as specified by Yamaha.
As the screw and lock nut rocker arm type valve train wears and starts to clatter the valve openings (lift) become smaller which improves low rpm port velocity and you will start to notice better low rpm power with an accompanying drop off in high rpm power. This is pretty much a fail-safe valve system, the valve lift gets smaller and performance will diminish until it becomes unacceptable, with no harm.
As shim/bucket valve trains wear valve lift increases resulting in lower port velocity at low rpm causing a drop in power and the higher lift increases breathing at high rpm allowing higher power. At least right up until the valves don't fully seat, then as Fred sez, the valve flute and/or the cylinder head valve seats start to silently burn. Check those valves as specified by Yamaha.