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its way into the needle valve. A gasolene-strainer should be placed between the fuel tank and carburetor, but even this will not always prevent foreign matter from working into the valve. Practically all float-feed carburetors have a drain cock at the bottom of the chamber, and this should be frequently opened after the motor has stood idle for some time and the gasolene allowed to run off. If the gasolene thus drained is caught in a glass bottle or similar vessel you will be surprised to find how much water or dirt frequently drains off. On multiple-cylinder motors it is often customary to use a single carburetor with the intake pipe or manifold attached to the several cylinders. This system often works to perfection, but in other cases a separate carburetor attached to each cylinder proves far more satisfactory. Frequently on two-cycle, two-port motors apparent carburetor trouble is due to the check-valve spring being either too weak or too stiff. It usually pays to have several springs of varying strengths on hand and by experimenting with these excellent results may often be obtained when it is impossible to get satisfactory operation through carburetor adjustments alone.

A very important part of the motor is the cooling system. In the case of air-cooled motors a simple fan driving the cool air across the motor is all that is required, and this is so simple and so easily watched that any tendency to overheat can be easily attended to. In the case of water-cooled motors the system is far more complicated, and overheating may be caused by some portion of the cooling system failing to operate where it is very difficult to locate it. A very simple system of

water-cooling used in many stationary motors, and in some vehicle motors, depends upon the well-known fact that hot water rises, and by placing a hopper, or radiator, at an elevation slightly higher than the motor

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and connecting it to the water jacket the water continually flows through the motor about the cylinder. This system does not prove very satisfactory with twocycle motors, as in this type of engine the explosion at every revolution tends to heat the cylinder walls far

faster than in the four-cycle motors where an explosion only occurs at every other revolution, thus allowing the motor to cool off appreciably between the explosions. Usually a pump of some sort is used to force the water through the jacket and around the cylinders. This pump may be either of the plunger type (Fig. 64), of the gear type, or of the true rotary type. The plunger type of pump is very satisfactory where clear water is

Fig. 65.–Rotary Pump

used, but in marine use it frequently becomes clogged by bits of seaweed, shells, gravel, or other matter becoming wedged between the check valves (Fig. 64, A, A) and their seats. This is readily removed by unscrewing the check-valve caps (Fig. 64, B, B). In addition to this trouble the plunger pump

is usually noisy and requires packing around the plunger (Fig. 64, C, C) in order to keep the pump from sucking air. Even when the plunger packing is as tight as it

can be made without offering great resistance, there is apt to be quite a little leakage of water which is extremely objectionable. In vehicle use the impracticability of keeping this type of pump tight has led to the almost universal adoption of some form of rotary pump. The true rotary pump is a very simple affair and may be driven by belt, cog-wheel, sprocket-and-chain, or by similar mechanical means; it has the advantage of operating noiselessly and, moreover, can be placed at

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some distance from the engine or where most convenient. A common form of this pump is shown in Fig. 65, but the shape of the paddles or interior fans varies with different makes. This style of pump circulates a large quantity of water at low pressure, and where a longer pipe line is used or where there is danger of dirt or other matter partially clogging the circulating system the gear pump is preferable. The Lobee gear pumps are typical of this class of circulating pumps, and one of these is illustrated in Fig. 66. These pumps may be driven in either direction, but the water will

always flow in the direction of revolution as indicated by the arrows in Fig. 67, A, B. For this reason a check valve should be placed between the engine and pump when a motor is frequently reversed, for otherwise air may be drawn into the intake pipe and an air-lock formed which will prevent the pump from working satisfactorily until the air is forced out and the pump primed,

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Fig. 67.-Flow of Water in Gear Pump

by pouring or forcing water through it and the pipes. Marine engines should always be provided with two check valves on the water pipe, one between pump and intake in boat and the other between the pump and water jacket (Fig. 38, CV). These prevent the water from flowing back from the cylinder through the pump, thus causing air in the pipes, and also prevent the boat

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