Burned, Sticking or Broken Valves

Weak valve springs.
Improper valve lifter clearance.
Improper valve guide clearance, or worn guides.
Out-of-round valve seats, or improper valve seat width.
Deposits or gum formation on valve stems, seats or guides.
Warped valves or faulty valve forgings.
Exhaust back pressure.
Improper spark timing.