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.
ENGINE STARTS; BUT STOPS
When an engine starts readily but quickly slows down and stops, the reason is almost always an insufficient supply of fuel. An obstruction in the pipe may prevent the fuel from flowing fast enough to keep the carburetor bowl filled when the engine is running; when the engine starts, the fuel is sucked out of the spray nozzle faster than it comes in through the float valve, so the carburetor is soon drained and the engine stops. The bowl then fills, only to be sucked dry again when the engine is next started.
This difficulty is caused by dirt in the fuel, which collects in the strainer or the fuel pipe. The strainer is so arranged that it may be easily drained and cleaned; to clear out the pipe, shut off the fuel at the tank, disconnect the pipe at both ends, and blow through it.
The strainer should be drained every day; it is sufficient to open the strainer drain cock for two or three seconds.
Most of the troubles due to dirt in the fuel will be avoided if the fuel is strained when filling the tank.
Another thing that will bring an engine to a stop is the clogging of the vent holes in the tank filler cap and in the top of the carburetor bowl. These holes should be clear, so that air can enter to replace the fuel that is used; if air cannot enter the fuel will not flow, and the tank is then said to be air-bound.