An engine will not deliver full power if it is run on a retarded spark. A loss of power from this cause will be accompanied by general overheating of the engine.
ENGINE STOPS
The manner in which an engine stops will indicate the reason for it.
A failure of the ignition system that stops the formation of current, like the sticking of the circuit breaker lever, will cut off all explosions instantly; the engine will stop abruptly. An engine will not stop abruptly from any fault with the mixture; with mixture trouble the explosions will become weaker and weaker until they cease.
If an engine stops through a failure of the lubrication or cooling systems it will be intensely hot, which will not be the case if the fault is with carburetion or ignition.
A running engine will not be brought to a stop by a loss of compression.
ENGINE MISSES
A steady or irregular miss in one cylinder is usually due to the spark plug’s being cracked or dirty. Carburetor trouble will affect all the cylinders; it cannot affect one cylinder only, and missing in one cylinder may be put down as ignition trouble. In this case ignition trouble does not mean magneto trouble, for if the magneto produces sparking current for one cylinder it will produce it for all. Therefore ignition trouble in only one cylinder is in those parts of the ignition system supplying that cylinder; that is, in the spark plug or in the spark plug cable.
A less likely cause for missing in one cylinder only is poor compression. It is usually the case that if compression is poor in one cylinder it is poor in them all, but a broken valve or piston ring or a weak valve spring will weaken compression in one and not in the others.
A cylinder that misses is cooler than the others, and can be located by feeling. It can also be located by short-circuiting the spark plugs one at a time; this will make no difference in the dead cylinder, but when the spark plug of an active cylinder is short-circuited the speed of the engine will drop.