Tractor Principles / The Action, Mechanism, Handling, Care, Maintenance and Repair of the Gas Engine Tractor

by Roger B. Whitman

FUEL SYSTEM AND CARBURETOR

The operation of a carburetor depends on so many things that no exact instructions for its adjustment can be given. The best that can be done is to give a general idea of the requirements, and to outline a plan by which the adjustment can be arrived at.

The many makes and designs of carburetors and vaporizers that are used on tractors have different kinds of adjustments; on most of them the only adjustment is the needle valve that controls the fuel, but some also have adjustable air valves. In any case, the manufacturer’s instruction book should be studied for the understanding of the particular carburetor in question.

The first step in adjusting a carburetor is to get the engine running. The needle valve should be closed, and then opened enough to give a mixture on which the engine will start; on many carburetors this will be about one and one half turns. The engine should then be primed; that is, a little gasoline should be put in the cylinder, which may be done with a squirt can.

When the engine is running, and is well heated, the needle valve should be gradually closed until the engine begins to miss, and to send jets of flame out of the carburetor, or little explosions occur in the carburetor. These are signs of a thin mixture, and the needle valve should be gradually opened to make the mixture richer. The engine will run more steadily, and will pick up speed until the mixture becomes too rich, when it will choke and black smoke will come out of the exhaust.

The positions of the needle valve for a mixture that is too thin and one that is too rich have thus been found, and it remains to set it at that point between at which the engine runs most steadily and at the best speed.

With adjustable air valves it is usual to adjust for idling, that is, the slowest speed at which the engine will run steadily without load, and then to make any necessary additional adjustment for full speed and power.

If a carburetor cannot be adjusted by following the usual methods, trouble may be looked for, and this may be in the carburetor itself, in the fuel supply, or in the intake manifold, taking for granted, of course, that the engine is in proper condition and that the ignition system is operating correctly.

Dirt under the float valve will prevent the valve from seating, and the level in the float chamber will be too high, so that the mixture is too rich. Lifting the valve from its seat will let fuel rush through, and loose particles will thus be washed away. If dirt is ground into the valve and seat, or if these parts are worn, the valve must be reseated, which is done by turning the valve against its seat with light pressure, the end of the valve being gently tapped with a light hammer. Under no conditions use a grinding compound, for the particles would become imbedded in the soft metal and would ruin the valve.

Other causes of flooding are a bent valve, the sticking of the float pivot, and the soaking of fuel into the cork float, which is thereby made too heavy to float properly. The remedy is to dry it, and then to give it three coats of shellac.

A frequent cause of trouble is dirt in the pipe from the tank to the carburetor. While there may not be enough dirt to prevent the engine from running slowly, it is sufficient to prevent the flow of sufficient fuel for full power. A strainer is always provided, and this should be drained every day; if this is not done frequently, dirt will work its way through.

A grain of sand in the spray nozzle will choke it, and every precaution should be taken to keep this from happening, as well as the other troubles that dirt brings. The best precaution is to strain the fuel through chamois leather, or, if this is not obtainable, through a very fine metal wire screen.

In fuel systems that use a pump, the sticking of the check valves, and the leaking of the pump through poor packing, will cut down the supply of fuel.

If air can leak into the carburetor or intake manifold, the proportions of the mixture will be altered. To test for leaks, run the engine, and with a squirt can squirt gasoline on the joints or other places that are suspected of leaking air. If there is a leak, the gasoline can be seen being sucked in.

Air must enter the tank to take the place of the fuel that flows out, and this is provided for by a small hole drilled in the tank-filling cap. If this hole becomes stopped up, the fuel will not flow, and the engine will come to a stop. There is a similar hole in the top of the float bowl of most carburetors, and this also must be kept open.

An engine is always started on gasoline, for that will form a mixture when it is cold. Before switching to kerosene the engine must be hot, and this will take several minutes of running on gasoline.

With a double carburetor, which has a separate fuel bowl and spray nozzle for each fuel, nothing more is required than the switching of one or the other into action; when the two parts have once been adjusted, they require no further adjustment. Carburetors that use the same spray nozzle for both gasoline and kerosene will require a readjustment when the switch is made, for, as kerosene is thicker than gasoline, it will require a larger opening for a sufficient quantity to pass. This readjustment is a slight opening of the needle valve on switching to kerosene, and an equal closing when gasoline is again used.

A few minutes before the engine is stopped the carburetor should be switched from kerosene to gasoline, so that when it is shut down the fuel bowl will contain gasoline and the cylinders gasoline mixture. This is done to make it possible to start the engine. If the engine is stopped on kerosene, it cannot be started if it has had time to cool. In such a case the fuel bowl must be drained of kerosene and filled with gasoline, and the engine must be cranked until the cylinders receive a clean gasoline mixture.

When an engine is working at full power on kerosene, it gets much hotter than would be the case with a gasoline mixture. Carbon particles in the cylinder, and projecting bits of metal, such as thin spark plug points or the edge of a screw thread, become so hot that they glow, with the result that they ignite the incoming fresh charge and cause preignition. The effect of this is to cause a pounding or knocking that is very noticeable. It is then necessary to use water, which is provided for in the carburetor.

Water has the effect of cooling the intensely heated parts, and only enough should be used to prevent preignition. When the knocking is heard, water should gradually be turned on, using no more than is necessary to stop the noise. Too much water will cause the engine to miss by collecting on the spark plug points, thereby preventing the passing of the ignition spark.

Hard water should not be used, for it will form scale, which will interfere with the action of the carburetor. Only soft water should be used, and preferably rain water.

Whenever the engine is stopped, the fuel valve at the tank should be closed to shut off the carburetor supply. If this is not done, the float valve will be the only thing that prevents the fuel from running out, and should the float valve leak, the fuel will be wasted.