Firing - Ignited by the spark, the compressed fuel explodes and forces the piston downward on its power stroke. Intake and exhaust valves are closed. Exhaust valves and seats are exposed momentarily to temperature as high as 3800 degrees F. Exhaust - The piston moves upward on its exhaust stroke and forces the burned fuel through the open exhaust valve. Intake valve is closed. Carbon, corrosion and wear and misalignment are inevitable products of normal engine operation.
Clean metal is a good heat conductor but carbon insulates and retains the heat.
This increases combustion chamber temperature and causes warping and burning. Unburned carbon residue gums valve stems and causes sticking in the guide. Deposits of hard carbon with sharp points projecting become white hot and cause pre-ignition and pinging. Carbon fouls spark plugs, which makes the engine miss and wastes fuel. Valve seats are held open and pitted when held open by carbon particles.