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_. ENGINE OVERHEATS An engine may overheat either because it produces more heat than the cooling system can take care of, or because the cooling system is not taking off all of the heat that it should. Running an engine with the spark retarded will cause it to overheat; so will a failure of the lubrication and an obstruction to the passage of the exhaust gases. If an engine has been taken down and overheats when it is reassembled, it may be that the magneto has been wrongly timed, and produces its spark too late. If an engine has been running properly but begins to overheat, the ignition cause will be the faulty setting of the spark control, or the slipping of the spark control rod. When an engine is run on kerosene, the oil in the crankcase must be frequently drained off and replaced with fresh oil. The reason for this is that part of the kerosene that goes to the cylinders does not vaporize and burn, but works its way past the pistons and into the crankcase, where it thins the lubricating oil. As the oil thins, it loses its ability to lubricate, and the engine begins to overheat. Anything that produces extra friction will cause overheating, as, for example, a wrist pin that works endways and rubs against the cylinder wall, or a tight bearing. For a cooling system to work properly it must contain a full supply of water, the passages must be clear, sufficient air must pass through the radiator, and the pump must be in proper condition.