Linc

So about 10000 kms ago I tore the left side head off to find a hole in the valve, put a new in it after dressing up the seat and lapping the valve, fresh valve seals, and away we go. Now, after 10000 kms, it's running crappy again, find out that it's the same cyl, rip the head off again,( I'm getting good at it, it took me an hour), low and behold, another burnt valve, same valve! Any thoughts,? It has HLA's, so what could be causing the valve to burn?