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.
A magneto that is kept clean and properly oiled rarely gives trouble, and it is a mistake to blame it whenever the engine runs irregularly or will not start. Its adjustments should be changed only when the other parts of the engine have been proved to be in good condition.
The working parts of a magneto are enclosed, and practically proof against dust. It should be wiped off frequently, and dust and grit should not be allowed to collect around the oil holes, for otherwise it will work into the bearings and damage them.
Dust and dirt are especially injurious to the circuit breaker, which should be frequently inspected and cleaned. Very little oil should be used on it, and this should be the light oil used for typewriters and sewing machines. A thicker oil will become gummy, and will prevent the free action of the lever.
If there is much sparking at the platinum points, so that they become corroded and rough, it is an indication that the condenser of the magneto is not operating as it should, for the object of the condenser is to prevent such sparking. The only remedy is to renew the condenser.
Rough points will spark more than smooth ones; should they get into this condition, they should be lightly filed with a file of the cut known as “dead smooth.” If this file cannot be obtained, pinch a strip of the finest sand paper—not emery paper—between the points, and draw it gently back and forth, smoothing down first one point and then the other. In smoothing platinum points the greatest care should be taken to make them flat and true to each other.
After smoothing the points they should be readjusted so that when they are separated by the cam they are from ¹/₃₂ to ¹/₆₄ inch apart.
A distributor made with a carbon brush that slides across the contacts will require wiping off at least once a month. Carbon dust will rub off the brush and collect on the face of the distributor; in the course of time this will cause a short-circuit. The distributor is always made so that it can easily be cleaned.
A magneto is timed to an engine so that when the spark control is fully retarded, the circuit breaker points are just separating as a piston goes over top center. The engine is cranked until one of the pistons is at top center; the magneto should be in position, but its coupling should be loose, so that the armature can be revolved. The spark control is retarded; that is, it is moved as far as possible in the direction in which the armature turns. The armature is then revolved in the direction in which it will be driven by the engine until it is seen that the contact points are beginning to separate; holding the armature, the coupling is then made fast.
It will now be found that the distributor brush is touching one of the contacts; that contact is to be connected with the spark plug of the cylinder that is at top center of the compression stroke. The following distributor contacts are connected to the remaining spark plugs in the order in which their cylinders fire.
Should the magneto be suspected of being out of order, the first test is to disconnect a wire from its spark plug, and support the tip ⅛ inch from the metal of the engine while the engine is cranked briskly; if a spark appears, it is evidence that the magneto is operating and that the trouble is elsewhere.
If there is no spark, repeat the test with the switch wire disconnected from the magneto. This wire and the switch form a circuit from the metal of the engine to the insulated part of the circuit breaker; when the switch is closed, or in the “off” position, this circuit is completed, and as the magneto current flows over it instead of over the regular sparking circuit, no spark is produced at the plug. It sometimes happens that the switch or wire is defective, and allows the current to take that circuit even when the switch is in the open or “run” position. If this is the case it will be shown by a spark on cranking the engine with the switch wire disconnected at the magneto, and no spark when it is connected.
If the switch and wire are all right, examine the circuit breaker to see whether the contact points are clean, and that they touch when the cam allows them to; touch the circuit breaker lever to see that it is free to move and that its spring is not broken. In some tractors the magneto is in such a position that the circuit breaker cannot easily be seen; in such a case hold a small mirror in front of the circuit breaker and examine the reflection.
If the circuit breaker is in good condition, examine the distributor to see whether it is dirty, or the brush broken; if these parts are all right, the trouble is of such a character as requires the magneto to be returned for repair.
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.
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.
Piston ring leaks are usually caused by the rings sticking in their grooves through the formation of carbon. To test for piston ring leaks, pour a half pint of cylinder oil into each cylinder, and crank the engine slowly. The oil will form a seal around the pistons, and if compression is then improved, the rings are shown to be at fault.
To free the rings, pour a few tablespoonfuls of kerosene into each cylinder, and spread it by giving the engine a few turns; after standing for an hour or so the carbon should be sufficiently softened to free the rings.
If the leakage of compression is due to the rings being worn and loose in their grooves, they must be replaced.
The most usual cause of compression loss is leaking valves. With its continual pounding against its seat, and the heat to which it is exposed, a valve and its seat will become rough and pitted, and will leak; when in this condition the valve must be ground.
A valve is ground by spreading grinding compound on the seat, and turning the valve against it. This requires the valve spring to be taken off; the exact method of doing this depends on how these parts are made.
If the valves are in a removable cylinder head, valve grinding is most easily done by taking the cylinder head to a bench. In many designs the valve seats are part of the cylinder casting, and the job is done on the tractor.
In grinding a valve the valve is not turned around in one direction only, for this would cut grooves in the valve and seat. To obtain smooth surfaces the valve should be given part of a turn in one direction, and then turned equally in the other direction; after every few turns the valve should be lifted and dropped to another position on the seat. In this way the grinding is made even all around.
Fig. 89.—Grinding Valve in Engine with Fixed Head
The best tool for valve grinding is a carpenter’s brace with a screw driver blade fitting the slot in the valve, as shown in Figure 89. This drawing illustrates a cylinder with a fixed head; the valve is reached by unscrewing the plug from the opening directly above it. When grinding valves in an engine of this design the opening between the valve pocket and the combustion space should be plugged with a rag or waste to prevent the grinding compound from getting into the cylinder.
With the valve grinding tool in position, swing the handle back and forth ten or twelve times; then lift the valve, place it in a new position, and repeat. The valve is lifted most easily by a light spring placed under the valve disk, as shown in Figure 89.
From time to time the valve disk and seat should be cleaned off and examined to see whether they are smooth and free from pits and scores. If they appear to be, make marks around the valve disk with a lead pencil, replace the valve, and give it a complete turn. If this wipes off the pencil marks all around the valve, the grinding is complete, and the valve may be replaced with its spring and spring retainer. It is not necessary to grind until the entire thickness of the valve disk and seat are smooth; a narrow band all around will make the valve tight.
After grinding, and before replacing the valve, all traces of the grinding compound should be wiped off, and great care taken that none of it gets into the cylinder, valve stem guide, or other working part.
Fig. 90.—Grinding Valve in Detachable Head
On an engine with a removable head containing the valves, the head may be taken to a work bench, which makes grinding easier. This is illustrated in Figure 90. On an engine in which the valve and its seat may be taken out, the seat may be clamped in a vise, as shown in Figure 91. With valves of either of these types, the grinding may be tested by turning the head or the seat so that the disk is down, and pouring in gasoline. If the valve is not tight, the gasoline will leak through, and grinding must be continued.
Fig. 91.—Grinding Valve in Detachable Seat
When a valve seat is very badly worn it must be redressed, which is done with a cutting tool to be obtained from the maker of the tractor, and illustrated in Figure 92. This has a stem fitting the valve stem guide which centers the tool and assures a true cut. If a seat is so worn as to need redressing, the valve will be in such bad condition that it must be discarded and a new one used. This must be ground in before the engine is run.
Grinding a valve lowers it in its seat, and usually makes it necessary to readjust the push rod. When an engine is cold there is a space of about ¹/₃₂ inch somewhere between the cam and the valve stem; in Figure 93, this space is shown to be between the valve stem and the rocker arm. As the engine heats up the valve stem lengthens, and this space permits it to do so.
Fig. 92.—Valve Seat Cutter
If the space is too small, the stem will come against the rocker arm or the push rod, and the valve will be held off its seat, causing a compression leak. If the space is too great, the valve will open too late and close too early. The space must therefore be carefully adjusted, and this is arranged for on practically all makes of tractor engines.
Fig. 93.—“Holt” Valve Arrangement
One-thirty-second of an inch is the thickness of a 10-cent piece; it should just be possible to slip a slightly worn dime into the space when the engine is cold.
By timing the valves is meant the setting of the cam shaft in such a position that the valves are opened at the correct point in the stroke. It is necessary to time the valves only when the cam shaft has been taken out and must be replaced. The principle of valve timing should be understood, however, in order to be able to tell whether an engine is timed correctly.
It will usually be found that the face of the flywheel bears letters and figures that are indicators of the timing of the valves. This arrangement on the E-B engines is shown in Figure 94. Two lines are cut in the face of the flywheel, one marked ex. cl. 1-4, which means exhaust valve closes, cylinders 1 and 4, and the other marked center 1-4, to indicate that the pistons in those cylinders are on center. A straight-edge is held against the finished surface of the housing and the crank shaft is turned to bring one of the marks in line with it; at that point the valves or pistons are as indicated by the lettering.
Fig. 94.—Valve Timing, Using Marks on Flywheel
Fig. 95.—Valve Timing
The flywheel is also marked with a dot to indicate the firing point. When the dot is in line with the straight-edge, ignition should occur with the spark control fully advanced.
Figure 95 shows the valve arrangement of the same engine, with the exhaust valve just closing; the point of the cam has passed under the lifter or push rod, and has permitted the valve to come to its seat, but is still holding the lifter against the valve stem.
To check the valve setting, hold a slip of tissue paper, such as a cigarette paper, in the space between the lifter and the valve stem, while the engine is cranked slowly. While the cam is holding the valve off its seat the paper will be pinched between the lifter and the valve stem and held firmly. At the instant when the paper is freed and can be moved, the valve is seated and the point of the cam is just passing from under; the proper mark on the flywheel should then be in line with the straight-edge.
As the cams for all valves are in one piece with the cam shaft, setting one valve sets them all and checking the setting of one checks the setting of all.
Before taking out a cam shaft, two adjoining teeth of its gear should be marked with a prick punch or a small cold chisel, and a similar mark should be made on the tooth of the crank shaft gear that comes between them. In replacing the cam shaft it is then necessary only to return the teeth to the same position. Timing gears are usually marked in this way by the manufacturers.
A kerosene lamp that is turned too high gives a dense black smoke that is composed of fine particles of carbon. A piece of paper held in the smoke is quickly covered with a deposit of carbon, commonly called soot, or lamp-black.
All fuel oils and lubricating oils contain carbon. When these oils burn in the cylinder, they produce carbon, much of which passes out of the exhaust, while the rest deposits on the valves and on all parts of the combustion space. This deposit hardens, and eventually makes trouble through causing preignition.
The deposit is rough, and the heat in the cylinder is sufficient to make the outstanding particles glow; they ignite the incoming charge, and cause preignition. The sign of carbon trouble is a sharp knocking in the cylinder, especially when the engine is under a heavy load. The sound is the same as that caused by too great an advance of the spark.
Carbon deposit can be greatly reduced by pouring a few tablespoonfuls of kerosene into each cylinder and cranking for a few turns to spread it to all parts of the combustion space. This will soften the carbon and much of it will be blown out when the engine is next started. Best results will be obtained if the kerosene is poured in after a run, when the engine is hot.
If the carbon deposit is too hard to be softened by kerosene, it can be removed by scraping. This requires the cylinder head to be taken off, when the deposit can be scraped and chipped with a screwdriver. Care should be taken to keep the carbon crumbs from getting into the cylinders, valve stem guides, or other places where it would cause wear.
In taking off the cylinder head the gasket should be handled carefully, and protected from denting and bending. A battered or bent gasket is a sure cause of compression leaks. In replacing a metal gasket, give it a coat of cylinder oil on both sides to improve its seating.
When replacing the cylinder head, set all of the bolts up a little at a time, instead of screwing some of them tight while others are loose. One bolt drawn tight may tilt the cylinder head slightly, and there will be a distortion when another bolt is tightened. This is avoided by setting up all of the bolts a little at a time.
Running on too rich a mixture, giving the engine too much oil, and not using an air cleaner in dusty work will carbonize an engine rapidly. Blue smoke at the exhaust is a sign that too much lubricating oil is being used; black smoke indicates too rich a mixture. Carbonizing can be greatly reduced by careful adjustment of the lubricator and carburetor.