[jDizzle]

scoach hit the nail right on the head.. wear in cylinder walls, and piston rings (what makes the seal between each piston and its cylinder's walls, causes reduced compression... valves seals can also wear and distort somewhat, also contributing to lost compression and over all performance... where your rockers touch the top of your valves, the clearance there also grows, resulting in a louder running engine (tapping) and the valves not being opened as far... valve adjustments are a simple way to gain power on an older motor, which involves putting the clearance just mentioned (between rockers and the top of valves) back to factory spec, to allow the valves to open fully, and the motor to run more quietly.