Is it a very loud ticking?
You're most likely trying to adjust the valves in the wrong part of the cycle.
Ignore what you've heard about "rotating 180 degrees", etc., just remove the valve adjustment cover and roll the engine over with the pull starter. Watch for the valves to cycle; when the intake valve closes you are now starting the "compression" stroke. Rotate the crank another 1/2 turn until you see the proper flywheel mark come into the viewing access hole down by the recoil starter. Set the valves.
2002 665 Prairie
FPS 12:1 pistons
FST Stage II Cams
2011 Stratos 186XT Bass Boat
Mercury Optimax 115hp
Minn Kota 70lb 24v trolling motor