Use New Technology
 

Bill at Advanced is exactly correct. We also use the newer type steel clad viton valve seals on all our heads and engines and they work trouble free and for a very long time. I would suggest getting away from the old valve seal technology Mercury was using on these motors back then and get to the new Viton seals on both intake and exhaust.
Most sticking valve issues come from valve guide to valve stem clearances being incorrect and to tight and this can be aggravated if improper oiling is an issue to the valve stems from overly tight "old school " seals.

Best Regards,
Ray @ Raylar