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.
To short-circuit a spark plug, take a wooden-handled screwdriver or other tool and rest the blade on the engine near the spark plug; then tilt until its shank is close to the spark plug terminal. The spark current will then pass to the metal of the engine by way of the tool instead of by the spark plug points. This is also a test of ignition, for a spark will pass between the terminal and the tool.
Irregular missing in all cylinders may be due to a fault at one of the parts of the ignition system that supplies them all; a dirty distributor, for instance, or a sticking circuit breaker lever, or rough platinum points. It may also be due to a clogged fuel line, which prevents the carburetor from getting a regular and sufficient flow.
Irregular missing will also be caused by loose ignition connections, and by loose switch parts.