CARBURETOR

The carburetor used on all Ford Tractor engines from the 1955 through the 1960 are basically the same type, with the same operating principles. The size of the main jets, however, vary depending on the size of the engine. The carburetors are the plain tube, up draft type. Interchange of carburetors on the manifold is impossible, as the carburetor stud spacing is different for each size carburetor.

A. Operation

To understand the operation of a carburetor it necessary to bear one basic fact in mind. The only function of a carburetor is to mix air and fuel and supply that mixture to the engine in the proper amount, and at the proper time. The Ford tractor carburetor is made up of four systems, see Figures 2 through S. These systems are: the float system, the idle system, the power fuel system, and the choke system. Each system except the float system supplies a fuel air mixture to the engine under different conditions of engine demand. Therefore, with all the systems functioning properly, the carburetor supplies the correct fuel air mixture through the entire range of engine operation. 1. The float system, as shown in Figure 2, controls the level of the gasoline in the carburetor fuel bowl, and admits fuel as necessary to maintain the proper fuel level throughout the operating range of the engine. As the fuel level lowers, the float lowers and the float valve (3), Figure 2, opens to admit more fuel to the float bowl. As the fuel reaches the operating level, the float raises, dosing the float valve, which shuts off the fuel supply. Actually, during normal operation, the float assembly positions so that the inward flow and the outward flow are about even.

The float system is a dual float mechanism, used in a float chamber or fuel bowl that almost completely surrounds the main discharge nozzle.

This construction permits operation of the engine at any angle up to 45 0 • 2. The idle fuel system, shown in Figure 3, controls the flow of fuel at idle speed and at slow speeds until the throttle is opened wide enough to allow the power system to operate. When the throttle plate (6), is in the idle position, it is slightly open, with the edge of the plate between the primary and secondary idle orifices (7) and (8). With the plate in this position, the air pressure at the primary orifice is lower than the air pressure in the fuel bowl chamber, due to manifold vacuum at the primary orifice. Therefore, fuel is forced from the fuel bowl into the idle passage (10), Figure 3. As the fuel travels through the idle fuel passage, it passes through the idle jet (11), to the point where it is combined with air entering through the idle fuel adjustment needle seat (12). The fuel is mixed with air again when it passes the secondary idle orifice as it travels through the idle fuel passage. The mixture that emerges from the primary idle orifice is rich, however, it is reduced to the correct proportion by the air passing the throttle plate.

flOAT VALVE

Figure 2- Floot System Figure 3- lole Fuel System

As the throttle plate is slowly opened from the idle position, the secondary idle orifice is gradually subjected to greater manifold vacuum, and as the manifold vacuum increases, the orifice ceases to bleed air to the idle fuel passage. Instead, fuel is fed from the idle fuel passage through the secondary idle orifice to the engine. With the throttle open wider than idle, a greater amount of air is admitted, necessitating a greater amount of fuel than can be supplied through the primary idle orifice. The additional fuel supplied through the secondary idle orifice maintains the correct fuel air ratio for the engine.

As the throttle is opened wider, the idle fuel delivery begins to fade and the power fuel system starts to function.

3. The power fuel system, shown in Figure 4, supplies fuel to the engine in the speed ranges beyond idle and slow speed.

As engine speed is increased, the air flow through the venturi (17), Figure 4, is increased. The increased air flow creates a vacuum at the nozzle tip (14). The pressure then is lower at the nozzle than it is in the fuel bowl chamber (9), Figure 3, and the accelerating well (16), Figure 4.

Fuel, therefore, flows from the fuel bowl, through the main adjustment needle and seat assembly (19) and out the nozzle (14) and into the air stream at the venturi. At the same time, the fuel in the accelerating well is forced through the nozzle air bleeds (15) in the nozzle. However, due to the size of the power jet and the restriction at the main adjustment needle seat, the fuel in the accelerating weH is soon exhausted, Bnd air enters the accelerating well through the nozzle air vent (20). Figure 4, The amount of air that can enter is limited by the size of the nozzle air vent. This air helps to atomize the fuel passing through the nozzle, and it helps to regulate the rate of discharge of the fuel fed from the accelerating well during acceleration. It also provides the correct mixture proportions for fuII throttle operation. When the throttle plate is opened suddenly to the wide open position, the fuel in the accelerating well is forced out through the nozzle air bleeds very rapidly, providing the extra rich mixture the engine needs to meet the sudden load. When the throttle is closed again, fuel fills the accelerating well, ready for the next acceleration.

4. The function of the choke plate is to restrict the air flow into the carburetor which causes a higher vacuum on the discharge nozzle. giving a richer mixture for starting purposes. The choke plate and choke system are shown in Figure 5.

A small movement in the choke plate causes a great change in the fuel air mixture.

The relief valve opens and then closes automatically depending on engine load and speed.

ACCHERATING WEU·16

Figure 4-Power fuel System figure 5- Choke System

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figure 6-CarouretorAdillstmenfs

When the engine reaches normal operating temperature, the choke plate must be fully open to assure maximum power and economy from the engine.

B. Adjustment

Three adjustments are provided on the carburetor, as shown in Figure 6. They are the main adjustment needle, the idle speed adjustment screw, and the idle fuel adjustment needle. Actually, there are three factors that control the conversion of the fuel air mixture into engine power. These factors are engine compression, ignition and correct carburetor adjustment. Correct carburetor adjustment cannot be obtained, however, unless engine compression and ignition meet specifications.

Before proceeding with the carburetor adjustment, the following preliminary steps should be performed to be sure the adjustments will be made correctly:

1. Set the idle speed adjustment screw so that the throttle plate is slightly open.

2. Make certain that the fuel tank shut-off valve is in the open position.

3. Pull the choke knob, turn on the ignition, start the engine and push the choke knob in until the engine runs smoothly.

4. After the engine has reached normal operating temperature, be sure the choke knob is pushed aU the way in, and proceed as follows: a, For the initial setting, before the engine is started, turn the idle fuel adjustment needle, Figure 6, in (clockwise) until it is seated lightly, then back it off approximately one turn. Start the engine and turn the idle fuel adjustment needle in (clockwise) until the

engine begins to "roll", then back it off slowly. until the engine is running smoothly, b. With the hand throttle in the closed position, tum the idle speed adjustment screw, Figure 6, until the engine idle speed is 450 to 475 rpm on the Proof-Meter. On new engines it may be necessary to set the idle slightly higher to prevent stalling. c. The power fuel adjustment or main adjustment needle, Figure 6, initial setting, is 1-1-% turns open, (counterclockwise).

The final adjustment should be made in the field With the engine running at governed speed under full load, turn the adjustment needle in (clockwise) after the engine power just starts to drop off, then turn the screw out (counterclockwise) until the power picks up and the engine runs smoothly.

C. Removal

1. Turn the fuel tank shut-off valve to the "OFF" position.

2. Loosen the air intake tube clamps at the carburetor and pull the hose off the carburetor. 3. Disconnect the fuel line at the sediment bowl.

4. Disconnect the governor and throttle rod linkage at the carburetor,

S. Remove the two nuts and lock washers that attach the carburetor to the manifold. Remove the carburetor and clean the gasket mounting surfaces,

6. Clean the outside of the carburetor with cleaning solvent to permit an external examination for damages or defects.

D. Disassemhly

L Loosen the bail wire and remove the sediment bowl.

2. Remove the fuel filter screen from the top of the sediment bowl cover,

3, Remove the main adjustment needle assembly, 4. Remove the carburetor bowl, screws, and lock washers. Separate the cover from the bowl. S. Remove the fioat, bowl gasket, venturi, and fioat valve from the cover, Figure 7. Remove the fioat valve seat and gasket with a screwdriver that is wide enough to engage both slots in the valve seat.

MAIN ADJUSTMENT NEEDLE

lEVER PIN ""- tOLE JET

VE AND SEAT I SEAT VALVE GASKET ASSEM6l Y TUllE STUDS

CHAMBER COVER GASKET

FLOAT AND lEVER ASSEMBLY

MAIN NOZZLE

CHOKE PLATt

Fi9IJre 7-Carburetor Disa$Sembled

NOTE: Later model carburetors incorporate a float valve support spring and support, Figure 7.

6. Remove the idle jet. 7. Remove the idle fuel adjustment needle and spring.

8. Mark the throttle plate so it can be reassembled in the same position, then remove the throttle plate screws, throttle plate and throttle shaft and lever assembly.

9. Remove the throttle shaft packing and retainer with a small punch.

10. Remove the main discharge nozzle and gasket.

11. Remove the drain plug from the bottom of the fuel bowl. 12. Remove the choke plate screws, choke plate, and the shaft and lever assembly. 13. Remove the choke shaft packing retainer and the packing.

E. Inspection and Cleaning

1. Clean all parts with a good solvent. Be sure all channels and orifices are clean. Discard jets showing signs of wear or corrosion.

IMPORTANT: Never use a wire or small drill to dean out orifices. Even a small change in the size oj a hole will affect carburetor operation.

2. Discard the float valve and seat if they are grooved or corroded. Always use an entire float valve and seat assembly, as they are serviced in matched sets.

3. Discard the idle fuel adjustment needle if it is grooved or corroded. 4. Inspect the throttle shaft and lever assembly for wear. If the shaft is loose in the bore, a new assembly should be used. 5. Inspect the carburetor fuel bowl and bowl cover for cracks or distortion. Check the float for leaks, cracks or distortion.

F. Assembly

A disassembled view of the carburetor is shown in Figure 7.

1. Install the retainer and new packing in the fuel bowl. Tap the retainer lightly until it is flush with the casting.

2. Install the throttle shaft in the bowl making sure the idle speed adjustment screw is to the rear of the stop. Place the throttle plate in operating position and install the screws and lock washers.

The angle number of the throttle plate should face toward the rear of the carburetor.

3. Instan the idle jet, Figure 7.

4. Instan the idle fuel adjustment needle and spring. Turn the needle in until it just contacts the seat, then back it off one turn for the initial adjustment.

S. Install a new gasket and the float valve seat using a screwdriver that is wide enough to contact both slots in the seat. Install the float valve, and fuel bowl gasket, then install the float and lever assembly.