Carburetor problems are among the most difficult internal combustion engine malfunctions to diagnose. If you have a carburetor problem, read the description of carburetor systems in the beginning of this section. Consider which system or combination of systems are in operation when the problem occurs. Some troubleshooting tips are given in the system operation descriptions.
The most reliable way for a nonprofessional to diagnose a bad carburetor is to eliminate all other possible sources of the problem. If you suspect the carburetor is the problem, perform the adjustments given in this section. Check the ignition system to ensure that the spark plugs, contact points, and condenser are in good shape and adjusted properly. Check the emission control equipment following the instructions given in Emission Controls of this information. Check the ignition timing adjustment. Check all vacuum hoses on the engine for loose connections, splits or breaks. Make sure the carburetor and intake manifold attaching bolts are tightened to the proper torque.
If you do determine that the carburetor is malfunctioning, and the adjustments in this section don't help, you have three alternatives: you can take it to a professional shop and let them fix it, you can buy a new or rebuilt carburetor to replace the one now on your vehicle, or you can buy a carburetor rebuilding kit and overhaul your carburetor.
See Figure 1Fast Idle
During cold engine starting and the engine warm-up period, a specially enriched fuel mixture is required. If the engine fails to run properly or if the engine over-revs with the choke closed in cold weather, the fast idle system should be checked and adjusted. This is accomplished with the carburetor installed.
- Run the engine to normal operating temperature.
- Connect a tachometer according to the tool manufacturer's instructions.
- Disconnect and plug the hose from the fast idle unloader.
- Shut the engine OFF , hold the choke valve closed, and open and close the throttle to engage the fast idle cam.
- Start the engine, run it for one minute. Fast idle speed should be 2300-3300 rpm (manual transaxle) or 2200-3200 rpm (automatic transaxle).
- Adjust the idle by turning the fast idle screw.
See Figures 2 and 3
Poor fuel combustion, black sooty exhaust, and fuel overflow are indications of improper float level. Lean running may also be a symptom, although you should also check for such causes as jets blocked by dirt and vacuum leaks.
- Make sure the vehicle is level. Start the engine and run it until it is warm. Remove the air cleaner and move it aside, leaving as many hoses connected as possible. Snap the throttle open so the engine accelerates to 3000 rpm several times, then allow the engine to idle.
- Wait until the fuel level stabilizes, and then check its location in the inspection window. If it is not centered, use a small screwdriver to adjust the fuel level, turning the screw no more than 1 / 8 turn every 15 seconds (so the fuel level will stabilize). When fuel level is correct, it is a good idea to put a dab of white paint near the screw to indicate that fuel level has been properly set. Stop the engine and replace the air cleaner.
See Figure 4
- Remove the air cleaner assembly to provide access.
- Check that the cable free-play (deflection) is 4.0-10.0mm (0.157-0.394 in.) This is measured right before the cable enters the throttle shaft bell crank.
- If deflection is not to specifications, rotate the cable adjusting nuts in the required direction.
- As a final check, have a friend press the gas pedal all the way to the floor, while you look down inside the throttle bore checking that the throttle plates reach the Wide Open Throttle (WOT) vertical position.
- Install the air cleaner.
See Figure 5
- Start engine and allow it to idle.
- The choke valve should start to open and fully at normal operating temperature.
- If not, disconnect the White/Blue wire from the choke cover and check for battery voltage with engine running.
- If no voltage at wire, check for open circuit in wire to voltage regulator connector and check alternator.
See Figures 6 and 7
- With the engine cold, remove the air cleaner.
- Open and close the throttle fully to let the choke close. The the choke valve should close completely.
Above about 82°F (28°C), the choke will not close completely, but should still close to less than1/8in. (3mm).
- If the choke closes properly, go on to the fast idle unloader test.
- If the choke does not close properly, spray linkage with carburetor cleaner and check again (use a spray can with an extension on nozzle to reach linkage).
- If the choke still does not close properly, remove choke cover and inspect linkage for free movement. Repair or replace parts as necessary. Then, reinstall cover and adjust so index marks line up and retest.
- If the choke still does not close properly, replace cover.
See Figure 8
- With the engine cold, disconnect the two hoses from fast idle unloader.
- Open and close throttle fully to engage fast idle cam.
- Start engine. Engine should run at fast idle.
- If engine does not run at fast idle, remove choke cover and check operation of fast idle cam.
- Connect a vacuum pump to inside fitting of unloader and apply vacuum. Idle should drop.
- If idle speed does not drop, check unloader for leaks, blockage or damaged diaphragm. Remove choke cover and check unloader rod for free movement. Repair or replace as necessary.
- Reconnect hoses.
- When engine warms, engine speed should drop below 1400 rpm as unloader pulls internal choke linkage off fast idle cam.
- If fast idle does not drop below 1400 rpm, disconnect the two unloader hoses and check that vacuum is present.
- If vacuum is present, check the unloader for leaks or blockage. Remove choke cover and check the unloader rod for free movement. Repair or replace parts as necessary. If there is no vacuum at inner fitting, check for vacuum at choke opener and thermovalve A. If there is no vacuum at outer fitting, check thermovalve A (for location, refer to the drawing accompanying the description of this subsystem.).
See Figures 9, 10, 11 and 12
- With the engine cold, disconnect the choke heater wire.
- Open and close the throttle fully to let the choke close.
- Start the engine. The choke valve should partially open.
- If the choke does not partially open, check the linkage for free movement, repair as necessary and retest.
- If the choke valve still does not partially open, check the choke opener diaphragm by removing the two screws and attaching a hand vacuum pump to hose fitting. Hold finger over the orifices in the opener while applying vacuum to pull opener rod in completely, then stop. If the rod will not stay in, replace the opener. If the rod stays in check vacuum port in carburetor and clean out if necessary.
- If the coolant temperature is below about 57°F (14°C), tab A on the choke opener lever should not be seated against the carburetor. If tab A is seated, disconnect the choke opener hose No. 18 from the upper fitting. If tab A comes off seat, check vacuum line No. 18 to the thermovalve for blockage and check that the thermovalve is open. If tab A does not come off its seat, press down on choke opener lever until it unseats. If it won't stay unseated, clean out the choke opener fitting with a 0.5mm drill bit and retest.
- If coolant temperature is above about 77°F (25°C), tab A on choke opener lever should be seated against carburetor.
- If tab A is not seated, check vacuum line No. 18 hose from upper fitting, for leaks and check that thermovalve A is closed. If tab A is seated, reconnect choke heater wires.
- Disconnect the No. 26 vacuum hose from the choke opener and connect a vacuum pump to the hose and apply vacuum.
- Vacuum should remain steady. Turn ignition switch to START with engine temperature below 104°F (40°C) vacuum should bleed off. If not go to the "Cranking Leak Test" testing procedure.
See Figure 13
- Disconnect the secondary diaphragm vacuum hose and attach a spare piece of hose between the diaphragm and a vacuum pump.
- Open the throttle valve fully and apply a vacuum. Check that the diaphragm rod moves as vacuum is applied and that vacuum then remains steady.
- If vacuum does not hold or rod does not move, first check hose for proper connection and condition, then replace diaphragm.
- With the engine cold (water temperature below 122°F or 50°C), disconnect vacuum hose from 3-way joint, connect a vacuum pump and apply vacuum. Vacuum should not hold.
- If vacuum holds, first make sure hose is not clogged, then replace thermovalve D.
- After the engine has warmed up, disconnect vacuum hose from the 3-way joint, connect a vacuum pump and apply vacuum. Vacuum should hold.
- If vacuum does not remain steady, check the hose for proper connection and that it is not clogged and replace thermovalve D.
- Disconnect the vacuum hose from the 3-way joint and connect to a vacuum pump/gauge. Apply vacuum. Vacuum should not hold.
- If vacuum is present, check hose, 3-way joint and clean vacuum port.
See Figures 14, 15 and 16
This procedure is not necessary unless the linkage has been bent, the choke opener was replaced or the car has poor cold starting. It can be performed while the engine is either hot or cold.
- Remove choke cover.
- While holding choke valve closed, open and close throttle fully to engage choke and fast idle linkage.
- Disconnect the choke opener hose from the steel tubing manifold. Attach a check valve to the hose. Then pressurize the choke opener with compressed air, 15-85 psi (103-586 kPa), to hold bleed valve closed.
- Push the choke opener lever towards opener until lever stops (opener rod seats against pressurized bleed valve), then pull choke drive lever down against opener lever (to take all free play out of linkage) and measure clearance between choke valve and casting.
- Adjust clearance by bending tab D.
- Remove the check valve, and reconnect the choke opener hose.
- Hold both levers together, then push levers toward diaphragm again until lever stop (tab A on opener lever seats against carburetor). Measure the clearance at choke valve.
- Adjust clearance by bending tab A.
- While still holding opener lever tab A against the seat, release choke drive lever and measure clearance at choke valve (tab C on drive lever should stay seated against spring loop; if not, repeat Step 2 and recheck).
Third stage clearance should be:
- Adjust clearance by bending tab C.
See Figures 17 and 18
- Disconnect hose No. 28 from the lower fitting of the choke opener, attach a hand vacuum pump to the hose and draw vacuum. Vacuum should remain steady. If the vacuum remains steady go to Step 2. If the vacuum drops, check hose No. 28 for leaks or cracks, if the hose is ok, replace the cranking leak solenoid valve.
- Turn the ignition switch to START , the vacuum should be released. If the vacuum is released the test is complete. If the vacuum is not released, check for voltage at the cranking leak solenoid valve (blue/white wire at control box No. 1) with the ignition still in the START position.
- If there is no voltage, check the wiring and the fuses. If there is voltage check hose No. 28 for blockage. If the hose is ok, replace the cranking leak solenoid valve.