Valves tend to make the same tapping noise at all speeds when they are out
of adjustment. The tapping is of course faster at higher rpm's. It is also
unusal for hydralic valve lifters to need adjusting. Do you have noise at
other speeds or only at high speed? Other problems with the lifters or oil
delivery can show up as lifter tapping.

The adjustment is pretty simple. With the engine off turn the crank so that
the valve to be adjust is completely closed. A number of valves can be
adjusted between turns of the crank. You do want to make sure the valve is
fully closed, not even slightly on the cam lobe ramp up or down. Back out
the valve nut, turn the nut in slowly while twisting the push rod with your
fingers. When you feel resistant to turning the rod you are at zero lash.
Now turn the nut another full turn in to center the lifter plunger. Some
people only turn a 1/2 or 3/4 turn, you can get a slightly longer duration
with that trick but risk it getting out of adjustment.

"B. Alexander" wrote in message
...
Hi,
I would like to know the proper way to adjust the intake/exhaust valves on
my Mercruiser Alpha One 3.0L inboard, they are making a loud clacking
sound
when the engine is at full speed.
Thanks in advance.