HI,

probable valve seals or guides, have you done a compression test,
leakage test would be better, than normal comp test. is the head off.
if so fill the chamber with parrifin with the cyl in question valves closed
and see if the parrifin escapes through the ports this doest necessary
mean the valves are bent just leaking when closed, if it pisse's out
they may be bent, if not sure take the valve out and roll it on a glass
surface,

DAVE R.