See Figures 1, 2 and 3
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-barrel carburetor used on 4 cylinder engines have 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 2-barrels. But basically, all carburetors are similar, and if you understand a simple 2-barrel, you can use that knowledge to understand a 4-barrel. 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 Toyota models are conventional 2-barrel, downdraft 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 Figure 4
Float level adjustments are unnecessary on models equipped with a carburetor sight glass, if the fuel level falls within the lines when the engine is running.
There are two float level adjustments which may be made on Toyota carburetors. One is with the air horn inverted, so that the float is in a fully raised position; the other is with the air horn in an upright position, so that the float falls to the bottom of its travel.
The float level is either measured with a special carburetor float level gauge, which comes with a rebuilding kit, or with a standard wire gauge.
- Turn the air horn upside down and let the float hang down by its own weight.
- Using a special float gauge (available at your local dealer), check the clearance between the tip of the float and the flat surface of the air horn. The clearance should be:
This measurement should be made without the gasket on the air horn.
- If the float clearance is not within specifications, adjust it by bending the upper (center) float tab.
- Lift up the float and check the clearance between the needle valve plunger and the float lip. Clearance on the 8R-C and 18R-C engines may be checked with a special float gauge or with a standard wire feeler gauge. 20R and 22R engines must only use the special float gauge. The clearance should be 0.039 in. (0.99mm) on all engines.
- If the clearance is not within specifications, adjust it by bending the lower float tabs (2).
See Figures 5 and 6
- Turn the carburetor cover upside down, then raise the float by hand.
- The clearance between the needle valve push pin and float tab should be 0.039 in. (1mm).
- Adjust it by bending the two outside float tabs.
OFF THE VEHICLE
See Figures 7 and 8
The fast idle adjustment is performed with the choke valve fully closed.
Adjust the gap between the throttle valve edge and bore to the specifications, where given, in the "Fast Idle Adjustment" chart. Use a wire gauge to determine the gap.
The chart also gives the proper primary throttle valve opening angle, where necessary, and the proper means of fast idle adjustment.
The throttle valve opening angle is measured with a gauge supplied in the carburetor rebuilding kit. It is also possible to make one out of cardboard by using a protractor to obtain the correct angle.Unloader
See Figures 9 and 10
The unloader adjustment is made with the primary throttle valve fully open. With the valve open, check the choke valve angle with a special gauge supplied in the rebuilding kit or with a gauge of the proper angle fabricated out of cardboard. The angle of the choke valve opening should be:
All angles, should be measured from the horizontal plane created by a closed choke valve.
To adjust the angle, bend the fast idle lever until the proper measurement is achieved.ON THE VEHICLE-1975-83 MODELS
See Figure 11
Disconnect the EGR valve vacuum line on 20R engines.
- Perform the idle speed/mixture adjustments as outlined in Engine Performance and Tune-up . Leave the tachometer connected.
- Remove the top of the air cleaner.
- Open the throttle valve slightly and close the choke valve. Next, hold the choke valve with your finger and close the throttle valve. The choke valve is now fully closed.
- Without depressing the accelerator pedal, start the engine.
- Check to see that the engine fast idle speed is 2400 rpm.
- If the reading on the tachometer is not within specifications, adjust the fast idle speed by turning the fast idle screw.
- Disconnect the tachometer, install the air cleaner cover, and connect the EGR valve vacuum line if it was detached.
See Figure 12
A reloader is used on the 8R-C engine to prevent the throttle valve from opening during automatic choke operation.
- When the choke valve is opened 45°deg; from the closed position, the reloader lever should disengage from its stop.
Angle "A" in the illustration should be 20°deg; when measured with a gauge.
- To adjust, bend the portion of the linkage where angle "A" was measured.
- When the primary throttle valve is fully opened, with the reloader in operating position, the clearance between the secondary throttle valve edge and bore should be 0.014-0.030 inch (0.355-0.762mm). Measure the clearance with a wire gauge and bend the reloader tab to adjust it.
- Fully open the choke valve by hand; the reloader lever should be disengaged from its stop by the weight on its link.
- Push the rod which comes out of the upper (choke breaker) diaphragm so that the choke valve opens.
- Measure the choke valve opening angle. It should be 40°deg;.
- Adjust the angle, if necessary, by bending the relief lever link.
- Apply vacuum to the larger of the two diaphragms.
- Check that the angle of the choke plate is 38°deg;, measured from the horizontal plane. Angle on 1983 22R engines is 42°deg;.
- If the angle is incorrect, adjust it by bending the choke breaker link.
- Apply vacuum to both of the diaphragms and check the choke plate angle again. It should be approximately 60°deg; (measured from the horizontal plane as before).
- If the angle is not within specifications, the choke breaker will require replacement.
See Figures 13, 14 and 15
When assembling the carburetor, turn the idle mixture screw the number of turns specified below. After the carburetor is installed, perform the appropriate idle speed/mixture adjustment as detailed in Engine Performance and Tune-up .
Idle mixture on the 1981 Celicas is preset at the factory and is not adjustable.Throttle Kick-up
See Figure 16
- Open the primary throttle valve. On 18R-C engines, the valve should be open 64°deg; from the bore; on 20R engines, the valve should be open all the way.
- The secondary throttle valve-to-bore clearance should be 0.008 inch (0.2mm). If not, adjust the clearance by bending the secondary throttle lever.
- Apply vacuum to the throttle positioner diaphragm.
- The throttle valve opening angle should be 16.5°deg; from the horizontal plane. If not, adjust it by turning the adjusting screw.
Check that the length of the pump stroke (length that the pump lever travels) is 0.154 inch (3.9mm). If it is not, it can be adjusted by bending the connecting link.
REMOVAL & INSTALLATION
- 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 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.
See Figures 17, 18, 19, 20 and 21
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 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 Toyota dealers:Minor Repair Kits:
Major 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 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.