To adjust the valves on the engine, remove the 4 screws in the valve cover and remove the valve cover. Remove the spark plug(s) from the engine.
Put a clean small screwdriver into the spark plug opening so you can tell when the piston is at the top of the stroke.
Rotate the flywheel until the piston is at top dead center and neither valve has any pressure on them. The push rods will be loose in the rocker arm. If one push rod has pressure on it, rotate the engine again and when it comes back to the top of the stroke neither push rod should have any pressure.
Use a feeler gauge to measure the clearance between the end of the push rod and the rocker arm.
The clearance on the intake valve should be .003.- 005 inches. The clearance on the exhaust valve should be .005-.007 inches.
If adjustment is required, loosen the allen set screw in the center of the nut on the rocker arm and adjust the nut to obtain the proper clearance. When you are through adjusting the clearance, tighten the set screw tight .
The intake push rod is aluminum, and the exhaust push rod is steel. This will help you determine which valve is which.
You could also have a worn lobe on the camshaft inside the engine. To test for this rotate the engine by hand and watch the rocker arms move. You should have about the same amount of lift on each rocker arm. If one or more rocker arms are barely moving, the lobe on the camshaft is worn.