Ignition trouble is usually in the spark plugs. The insulator cracks easily in many makes, which will permit the current to leak across without forming a spark; it is frequently the case that the crack does not show, and the best test is to replace the suspected plug with a plug that is known to be good. If the cylinder fires with one plug and not with the other, there is no question as to the cause of the trouble. The insulator of the plug must be kept clean, for a deposit of carbon on it will form a path by which the current can pass without forming a spark. A dirty plug can best be cleaned by brushing it with a stiff toothbrush dipped in gasoline. A carbon deposit can be softened by soaking the plug in gasoline for a few hours, and can then be brushed off more easily. The spark gap of a plug should be from ¹/₃₂ to ¹/₆₄ inch. After considerable use the points will be burned off, and the gap will become too wide; the points should then be bent to form a proper gap. Oil and grease will rot rubber, and the ignition wires should therefore be wiped clean. Oil-soaked cables will give trouble, and should be replaced with new ones. It is frequently difficult to locate a leakage of current. If the engine is misfiring and losing power, and a leakage of current through poor insulation is suspected, the easiest way to detect it is to run the engine in the dark. Leaks will show themselves by sparks, which are then easily seen. COMPRESSION In order to deliver its full power a gas engine must have good compression, and compression should frequently be tested by cranking the engine slowly and steadily with the ignition switched off. If compression is good, there will be a springy, elastic resistance that becomes greater as a piston approaches the end of a compression stroke, and that throws the piston outward as dead center is passed. Compression should be the same for all cylinders. If there is a leakage of compression, the only resistance will be from the bearings, and it will be the same for all parts of the stroke. A compression leak often makes a hissing noise that can be distinctly heard, and by which it can be located, but more often it makes no sound, and its location must be found by testing. The leak may be at any of the openings into the combustion space; at the valves, around the spark plugs or piston rings, or at the cylinder head gasket. To discover whether the gasket leaks, run gasoline along the line of the gasket joint with a squirt can while the engine is being cranked briskly; at a leaky place it will be sucked in or blown out. The same test should be made around the spark plug. The remedy is to reset the cylinder head, using a new gasket, and being sure that the surfaces are clean and free from grit.