PRE-IGNITION
Carbon deposits form a heat barrier and can be a contributing factor to preignition. Other causes include: An overheated spark plug (too hot a heat range for the application). Glowing carbon deposits on a hot exhaust valve (which may mean the valve is running too hot because of poor seating, a weak valve spring or insufficient valve lash).
A sharp edge in the combustion chamber or on top of a piston (rounding sharp edges with a grinder can eliminate this cause).
Sharp edges on valves that were reground improperly (not enough margin left on the edges).
A lean fuel mixture.
Low coolant level, slipping fan clutch, inoperative electric cooling fan or other cooling system problem that causes the engine to run hotter than normal.
