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. Once in the bore 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 (fuel/air circuits) in a carburetor that make it work:
The way these systems are arranged in the carburetor determines the carburetor's size and shape. It's hard to believe that the little single-barrel carburetor used on many 4 cylinder engines has all the same basic systems as the enormous 4-barrels used on V8 engines. Of course, the 4-barrels have more throttle bores (barrels) and a lot of other hardware you won't find on the little single-barrels. But, basically, all carburetors are similar, and if you understand a simple single-barrel, you can use that knowledge to understand a 4-barrel. If you'll study the explanations of the various systems, 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. Feedback carburettors found on late model vehicles require even less attention as the mixture is not adjustable, as part of regular service.
The carburetors used on Toyota models are conventional two-barrel, down-draft types similar to domestic carburetors. 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.
See Figures 1 through 6
On early model carburetors some adjustment service procedures must be modified. Use these service procedures as a general guide
Before making any adjustments to the carburetor, ALL of the following conditions must be met:
The curb idle is adjusted by turning the idle adjusting screw located on the rear of the carburetor. On most models, the knob has a knurled plastic head to make grasping easier. Correct idle speed for the carbureted engine is usually 650 rpm w/manual transmission and 750 rpm w/automatic transmission. Always refer to the underhood emission sticker for specifications as some models/engines may differ slightly.Fast Idle
See Figures 7 and 8
- Stop the engine and remove the air cleaner housing.
- Disconnect and plug the hot idle compensator hose to prevent rough idling.
- If so equipped, disconnect the hose from the Thermovacuum Switching Valve (TVSV) port M and plug the port. This will shut off the choke opener and EGR systems.
- Hold the throttle slightly open, (either move the linkage on the carburetor or pull lightly on the throttle cable) push the choke plate closed and hold it closed as you release the throttle.
The carburetor is now fooled into thinking it is performing a cold start-the choke is set and the various external controls are not functioning. These conditions duplicate cold start conditions.
- Start the engine but DO NOT touch the accelerator pedal or cable. (If you do, the choke will release and Step 4 will be needed again.)
- The correct fast idle speed is 3000 rpm. If adjustment is necessary, turn the fast idle adjusting screw at the lower rear of the carburetor. Always refer to the underhood emission sticker for specifications as some models/engines may differ slightly.
- Remove the plug and reconnect the hose to the M port of the TVSV, if so equipped.
See Figures 9, 10 and 11
- Disconnect the hose from the Thermovacuum Switching Valve (TVSV) port M and plug the port, if so equipped. Disconnect the vacuum hose from Throttle Positioner (TP) diaphragm A.
- Check that the TP is set at the first step; correct engine speed is:
If necessary, adjust the speed with the adjusting screw.
Make the adjustment with the cooling (radiator) fan OFF. Always refer to the underhood emission sticker for specifications. As some models/engines may differ slightly always follow specifications on emission sticker if none appear (the sticker is missing or damaged) follow the above
- Reconnect the vacuum hose to diaphragm A.
- Disconnect the hose from diaphragm B and plug the hose end.
- Check that the TP is set at the second step. The correct engine speed in this position is:
- Reconnect the vacuum hose to diaphragm B and check that the engine returns to normal idle within 2-6 seconds.
- Remove the plug and reconnect the vacuum hose to TVSV port M.
In order to correct float level adjustment a rebuild kit is necessary to supply the related gaskets. Refer to the instructions in the rebuild kit for the latest specifications
The float level is not externally adjustable. Removal of the air horn assembly or top of the carburetor is required. The engine should be cold during this procedure. All work is performed with the engine OFF .
The float in the carburetor controls the entry of fuel into the bowl of the carburetor (the bowl is simply a reservoir which keeps fuel available at all times). The function of the float is to react to the level of fuel in the bowl and open or close a valve, thus maintaining the correct amount of fuel. The principle is identical to the float in a toilet tank; when the correct level is reached, the flow is shut off.
The position of the float (and therefore the amount of fuel available) is critical to proper operation of the engine. If too little fuel is in the bowl, the engine may starve on sharp corners or on hills; too much fuel can literally lead to overflowing and flooding of the engine. To adjust the float level:
- Remove the air cleaner assembly and disconnect the choke linkage.
- Disconnect the accelerator pump connecting rod.
- Remove the pump arm pivot screw and the pump arm.
- Remove the fuel hose and union.
- Remove the eight air horn screws. Be careful to identify and collect the external parts attached to the screws, such as wire clamps, brackets and the steel number plate.
- Disconnect the choke link.
- Lift the air horn with its gasket from the body of the carburetor.
- Disconnect the wires at the connector.
- Remove the gasket from the air horn assembly. Invert the air horn so that the float hangs down by its own weight. Check the clearance between the float tip and the air horn. Adjust the float lip by bending it gently into position.
- Lift up the float, then check the clearance between the needle valve plunger and the float lip. If necessary, adjust the clearance by bending the outer part of the float lip.
If the float has become misadjusted, it may be due to the float filling with gasoline (hole in float). Give the float a gentle shake and listen for any liquid within. If the float contains fuel replace the float and reset the levels
- Install a new gasket onto the air horn.
- Place the air horn in position on the carburetor body and install the choke link.
- Install the eight screws. Make certain the brackets, clips and steel tag are reinstalled as well.
- Connect the fuel hose and union.
- Install the pump arm pivot screw and pump arm.
- Connect the pump arm connecting rod.
- Attach the choke linkage.
- Install the air cleaner. Start the engine and check carefully for fuel and/or vacuum leaks. The engine may be difficult to start; once it is running smoothly, recheck the fuel level in the sight glass.
REMOVAL & INSTALLATION
See Figures 12 through 18
Each fuel and vacuum line must be tagged or labeled individually during disassembly for correct installation. Never tilt the carburetor assembly during removal or installationEarly Models
- Disconnect the negative battery cable.
- Loosen the radiator drain plug and drain the coolant into a suitable container.
- Unthread 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 cars 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
- Installation is in the reverse order of removal.
- Start the engine and check for any leaks. Check the float level.
- Remove the air cleaner assembly.
- Disconnect the accelerator cable from the carburetor.
- If equipped with automatic transmission, disconnect the throttle position cable.
- Unplug the wiring connector.
- Label and disconnect:
- Carburetor vacuum hoses
- Fuel inlet hoses
- Charcoal canister hose
- Remove the carburetor mounting nuts.
- Remove the cold mixture heater wire clamp and lift out the EGR vacuum modulator bracket.
- Lift the carburetor off the engine and place it on a clean cloth on the workbench. If desired, the insulator (base gasket) may also be removed.
- Cover the inlet area of the manifold with clean rags. This will prevent the entry of dust, dirt and loose parts.
- Place the insulator on the manifold, making sure it is correctly positioned.
- Install the carburetor onto the manifold.
- Install the EGR vacuum modulator bracket. Clamp the cold mixture heater wire into place.
- Tighten the carburetor mounting nuts (always torque the mounting bolts evenly in steps).
- Reconnect the fuel inlet hose, the charcoal canister hose and the vacuum hoses.
- Engage the wiring connector.
- Connect the accelerator cable. If equipped with automatic transmission, connect the throttle position cable.
- Reinstall the air cleaner.
See Figures 19 and 20
A rebuild kit for a correct overhaul is necessary. Refer to the instructions in the rebuild kit for the latest specifications
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 in order 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 or allow them to air dry. Wipe clean all cork, plastic, leather, and fiber parts with a clean, lint-free cloth.
Blow out all passages and jets with compressed air and be sure that there are no 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 straightedge.
- 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 later.
Most carburetor manufacturers supply overhaul kits of 3 basic types: minor repair; major repair; and gasket kits. Basically, they contain the following:Minor Repair Kits:
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 over tighten, 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.