Like Easy said...
and if a valve clearance (or "lash") gets tight (below minimum spec.) the valve and/or valve seat in the head will burn & start leaking, especially the exhaust valves. Leaky valves equal lower compression and a top end rebuild (valve job).$$$
They burn because the valve is not closing fully thus allowing a flow of high temperature/high pressure gas to erode the metal.
And again as Easy stated, by the time you notice a loss of performance the damage is done. Hence the necessity of scheduled valve lash measurement and if needed, adjustment.
Too large a lash will lead to noisy valves and accelerated wear of valve train components, plus a decrease in horsepower :shocked:
I have checked mine twice (at 6500mi and 9500mi) and they needed a small adjustment each time to be right on spec for the way I want my bike to run. I set mine towards the smaller end of the range, knowing I am going to check them on a regular basis. A larger gap may allow a longer interval between services but hurt top end performance. The range of allowable lash enables a small amount of "tuning" to be done. You can vary the amount of time the valve is open(duration) by varying the lash. Smaller gap=longer duration, bigger gap=shorter duration.
Reducing the lash (smaller gap-longer duration) will increase top end (upper RPM power) horsepower and make for a quieter valve train. Almost like a "bigger" cam! :rawk:
Increasing the lash (bigger gap-shorter duration) can improve low end torque and acceleration, and easier starting, as well as a longer service interval :2tup:
I found that a bicycle nipple wrench works well to keep the small square end of the threaded adjuster from turning while you tighten the holding nut.