Spark plug in poor position.
Wrong timing of ignition system; too early, too late, or wrong order.
Dirty distributor, misdirecting the current.
Faulty Carburetion:
Pre-ignition, due to excessive carbon deposits.
Rich mixture, causing overheating.
Lean mixture, in conjunction with worn parts.
Faulty Lubrication:
Lack of oil.
Lubricants of poor quality.