Oh that the fun to do and most likely you find that it might your problem too.
Requires removing the head so the valves can be removed if the clearances have closed and that is fairly common on L heads (Side Valves).
Once the valves are out you file squarely the valve stem to increase the clearances. To close up you either install new valves and/or grind the valve seats. Be carefully as you can over do it quickly. I usually go for max clearances as they do close up over time from wear of the valve face and seat.
Got to go a turkey hunter just showed up and I got to hid....
