See Figures 1, 2, 3, 4 and 5
The carburetor is the most complex part of the entire fuel system. Carburetors vary greatly in construction, but they all operate basically the same way; their job is to supply the correct mixture of fuel and air to the engine in response to varying conditions.
Despite their complexity in operation, carburetors function because of a simple physical principle (the venturi principle). Air is drawn into the engine by the pumping action of the pistons. As the air enters the top of the carburetor, it passes through a venturi, which is nothing more than a restriction in the throttle bore. The air speeds up as it passes through the venturi, causing a slight drop in pressure. This pressure drop pulls fuel from the float bowl through a nozzle into the throttle bore, where it mixes with the air and forms a fine mist, which is distributed to the cylinders through the intake manifold.
There are six different systems (air/fuel circuits) in a carburetor that make it work; the Float system, Main Metering system, Idle and Low-Speed system, Accelerator Pump system, Power system, and the Choke system. The way these systems are arranged in the carburetor determines the carburetor's size and shape.
It's hard to believe that the 2-bbl carburetor used on 4 cylinder engines have all the same basic systems as the enormous 4-bbl carburetors used on V8 engines. Of course, the 4-bbl have more throttle bores ("barrels") and a lot of other hardware you won't find on the little 2-bbl. But basically, all carburetors are similar, and if you understand a simple 2-bbl, you can use that knowledge to understand a 4-bbl. If you'll study the explanations of the various systems on this stage, you'll discover that carburetors aren't as tricky as you thought they were. In fact, they're fairly simple, considering the job they have to do.
It's important to remember that carburetors seldom give trouble during normal operation. Other than changing the fuel and air filters and making sure the idle speed and mixture are OK at every tune-up, there's not much maintenance you can perform on the average carburetor.
The carburetors used on Nissan and Datsun pickups are conventional 2-bbl, downdraft types. The main circuits are: primary, for normal operational requirements; secondary, to supply high speed fuel needs; float, to supply fuel to the primary and secondary circuits; accelerator, to supply fuel for quick and safe acceleration; choke, for reliable starting in cold weather; and power valve, for fuel economy. Although slight differences in appearance may be noted, these carburetors are basically alike. Of course, different jets and settings are demanded by the different engines to which they are fitted.
REMOVAL & INSTALLATION
See Figures 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 and 16
- Disconnect the negative battery cable.
- Loosen the radiator drain plug and drain the coolant into a suitable container.
- Unscrew the mounting screws and remove the air filter housing. Disconnect all hoses and lines leading from the air cleaner.
- Tag and disconnect all fuel, vacuum, coolant and electrical lines or hoses leading from the carburetor.
- Disconnect the accelerator linkage from the carburetor. On trucks equipped with an automatic transmission, disconnect the throttle cable linkage running from the transmission.
- Remove the four carburetor mounting bolts and lift off the carburetor and its gasket.
Cover the manifold opening with a clean rag to prevent anything from falling into the engine.
- Install the carburetor, tighten the mounting bolts EVENLY IN STEPS and reconnect all linkage.
- Start the engine and check for any leaks. Check the float level. Road test the vehicle for proper operation.
See Figure 17
- With the engine cold, make sure the choke is fully closed (press the gas pedal all the way to the floor and release, or pull the choke knob out on early models with that system).
- Check the choke linkage for binding. The choke plate should be easily opened and closed with your finger. If the choke sticks or binds, it can usually be freed with a liberal application of a carburetor cleaner make for the purpose. A couple of quick shots from a spray can of this stuff normally does the trick. If not, the carburetor will have to be disassembled for repairs.
- The choke is correctly adjusted when the index mark on the choke housing (notch) aligns with the center mark on the carburetor body. If the setting is incorrect, loosen the three screws clamping the choke body in place and rotate the choke cover left or right until the marks align. Tighten the screws carefully to avoid cracking the housing.
See Figures 18, 19, 20 and 21
These carburetors utilize an electric choke which cannot be adjusted. If the choke is not functioning properly, perform the following tests and replace the defective parts.Choke Heater Circuit
- With the ignition off, check for continuity between leads A and B in the illustration.
- If continuity is found, the heater is good. If continuity is not found, check for shorts or open wires.
- With the engine at idle, check for voltage across A & B . A reading of 12 volts should be indicated. If not, check for a short or open circuit.
- Remove the relay, located on the right side of the firewall.
- Check for continuity between 4 and 5 ; and between 1 and 2 . Continuity should exist each time.
- Check for continuity between 1 and 3 . There should be none.
- Apply 12 volts across 4 and 5 . Continuity should now exist between 1 and 3 , but not between 1 and 2 .
- If all these conditions are not met, replace the relay.
On all models, make sure the throttle is wide open when the accelerator pedal is floored. Some models have an adjustable accelerator pedal stop to prevent strain on the linkage.Secondary Throttle Linkage
See Figure 22
All Datsun/Nissan carburetors discussed in this repair guide are two stage type carburetors. On this type of carburetor, the engine runs on the primary barrel most of the time, with the secondary barrel being used for acceleration purposes. When the throttle valve on the primary side opens to an angle of approximately 50 degrees (from its fully closed position), the secondary throttle valve is pulled open by the connecting linkage. The fifty degree angle of throttle valve opening works out to a clearance measurement of somewhere between 0.26-0.32 in. between the throttle valve and the carburetor body. The easiest way to measure this is to use a drill bit. Drill bits from size H to size P (standard letter size drill bits) should fit. If an adjustment is necessary, bend the connecting link between the two linkage assemblies.
See Figure 23
The fuel level is normal if it is within the lines on the window glass of the float chamber when the vehicle is resting on level ground and the engine is off.
If the fuel level is outside the lines, remove the float housing cover. Have an absorbent cloth under the cover to catch the fuel from the fuel bowl. Adjust the float level by bending the needle seat on the float.
The needle valve should have an effective stroke of about 0.0591 in. (1.5mm). When necessary, the needle valve stroke can be adjusted by bending the float stopper.
Be careful not to bend the needle valve rod when installing the float and baffle plate, if removed.
See Figures 24, 25 and 26
- With the carburetor removed from the vehicle, place the upper side of the fast idle screw on the 2nd step (1st step for 1977-82 engines) of the fast idle cam and measure the clearance between the throttle valve and the wall of the throttle valve chamber at the center of the throttle valve. Check it against the following specifications:
M/T means manual transmission. A/T means automatic transmission.
The first step of the fast idle adjustment procedure is not absolutely necessary.
- Install the carburetor on the engine.
- Start the engine and measure the fast idle rpm with the engine at operating temperature. The cam should be at the 2nd step.
- To adjust the fast idle speed, turn the fast idle adjusting screw counterclockwise to increase the fast idle speed and clockwise to decrease the fast idle speed.
See Figure 27
- Close the choke valve completely.
- Hold the choke valve closed by stretching a rubber band between the choke piston lever and a stationary part of the carburetor.
- Open the throttle lever fully.
With the throttle lever fully open, adjust the clearance between the choke valve and the carburetor body to the figure indicated:
1973-74: 0.1730 in. (4.4mm)
Make sure that the throttle valve opens completely when the carburetor is mounted on the engine.
See Figures 28, 29 and 30
The purpose of this device is to prevent the throttle from suddenly snapping shut. The dashpot has a plunger which extends when the throttle is closed suddenly. The plunger contacts a tab on the throttle lever and holds the throttle open slightly for a second, then closes the throttle slowly over the period of another second or so.
- Adjust the idle speed and mixture before making adjustments to the dashpot. Warm the engine to operating temperature, and connect a tachometer to the engine.
- Move the throttle lever by hand, and note the engine speed when the dashpot plunger just touches the throttle lever.
The engine speed should be as indicated.
1970-72: 1800-2000 rpm
- If not, loosen the locknut and turn the adjusting screw until the engine speed is in the proper range. Tighten the locknut. On 1978-79 models with air conditioning, a different dashpot is used. Adjustment is made by turning the screw on the throttle lever which contacts the plunger.
- Open the throttle and allow it to close by itself. The dashpot should smoothly reduce the idling speed from 2000 to 1000 rpm in about three seconds.
Efficient carburetion depends greatly on careful cleaning and inspection during overhaul, since dirt, gum, water, or varnish in or on the carburetor parts are often responsible for poor performance.
Overhaul your carburetor in a clean, dust-free area. Carefully disassemble the carburetor, referring often to the exploded views. Keep all similar and look-alike parts segregated during disassembly and cleaning to avoid accidental interchange during assembly. Make a note of all jet sizes.
When the carburetor is disassembled, wash all parts (except diaphragms, electric choke units, pump plunger, and any other plastic, leather, fiber, or rubber parts) in clean carburetor solvent. Do not leave parts in the solvent any longer than is necessary to sufficiently loosen the deposits. Excessive cleaning may remove the special finish from the float bowl and choke valve bodies, leaving these parts unfit for service. Rinse all parts in clean solvent and blow them dry with compressed air to allow them to air dry. Wipe clean all cork, plastic, leather, and fiber parts with a clean, lint-free cloth.
Carburetor solvent is available in various-sized solvent cans, which are designed with a removable small parts basket in the top. The carburetor choke chamber and body, and all small parts can be soaked in this can until clean. These solvent cans are available at most auto parts stores, and are quite handy for soaking other small engine parts.
Blow out all passages and jets with compressed air and be sure that there are not restrictions or blockages. Never use wire or similar tools to clean jets, fuel passages, or air bleeds. Clean all jets and valves separately to avoid accidental interchange.
Check all parts for wear or damage. If wear or damage is found, replace the defective parts. Especially check the following:
- Check the float needle and seat for wear. If wear is found, replace the complete assembly.
- Check the float hinge pin for wear and the float(s) for dents or distortion. Replace the float if fuel has leaked into it.
- Check the throttle and choke shaft bores for wear or an out-of-round condition. Damage or wear to the throttle arm, shaft, or shaft bore will often require replacement of the throttle body. These parts require a close tolerance of fit; wear may allow air leakage, which could affect starting and idling.
Throttle shafts and bushings are not included in overhaul kits. They can be purchased separately.
- Inspect the idle mixture adjusting needles for burrs or grooves. Any such condition requires replacement of the needle, since you will not be able to obtain a satisfactory idle.
- Test the accelerator pump check valves. They should pass air one way but not the other. Test for proper seating by blowing and sucking on the valve. Replace the valve if necessary. If the valve is satisfactory, wash the valve again to remove breath moisture.
- Check the bowl cover for warped surfaces with a straight edge.
- Closely inspect the valves and seats for wear and damage, replacing as necessary.
- After the carburetor is assembled, check the choke valve for freedom of operation.
Carburetor overhaul kits are recommended for each overhaul. These kits contain all gaskets and new parts to replace those that deteriorate most rapidly. Failure to replace all parts supplied with the kit (especially gaskets) can result in poor performance and a leaky carburetor later.
Most carburetor manufacturers supply overhaul kits in at least one of three basic types: minor repair; major repair; and gasket kits. Basically, they contain the following, and are available at most auto parts jobbers and Nissan dealers:
After cleaning and checking all components, reassemble the carburetor, using new parts and referring to the exploded view. When reassembling, make sure that all screws and jets are tight in their seats, but do not overtighten as the tips will be distorted. Tighten all screws gradually in rotation. Do not tighten needle valves into their seats; uneven jetting will result. Always use new gaskets. Be sure to adjust the float level when reassembling.