Before attempting a carburetor repair or service, perform a visual inspection.
- Look for obvious problems, missing parts, torn or damaged hoses.
- With the engine running, listen for hissing, which could indicate an air (vacuum) leak.
- An air leak is usually accompanied by a rough, higher idle and hesitation on acceleration.
Carburetor difficulties can also be indicated by such things as certain engine performance problems that are typically caused by improper mixture situations, excessive exhaust emission levels, poor idle or stalling, and/or a fuel problem.
Prior to diagnosing the cause of the difficulty, make sure the engine's compression, ignition system, and emission control devices are in satisfactory condition. Air/fuel mixture problems are most easily identified by testing the engine's exhaust, particularly by watching the O2 (oxygen) levels.
- The O2 should be observed when the engine is running at idle and at about 2,500 rpm.
- Make sure the engine is at its normal operating temperature and that the catalytic converter and O2 sensor are warmed up.
- At idle speeds, the O2 level should be between 1 and 4%. If the oxygen level is greater than that, a lean mixture or ignition problem is evident. If the readings are below 1%, the mixture is too rich.
- Now raise the engine's speed to 2,500 rpm.
- The O2 levels should be in the same range, except for engines equipped with a feedback carburetor.
- The exhaust from these engines should have no more than 1% O2 with no more than 1% CO.
- Readings higher than these indicate a problem with the mixture or the ignition system.
- Both of these systems should be tested further.
- Air/fuel mixture problems may also be evident when improper idle speeds or stalling occur.
- Many devices are involved in maintaining the proper idle speed of an engine.
- Begin diagnosis with a basic check of the idle speed to make sure the stalling or rough idle is caused by improper idle speed.
- Normally, idle speed adjustments are not possible on carburetors with electronic idle-speed control.
Engine Idle Speed Check
If idle speed adjustment is possible on the carburetor, be sure to follow the manufacturer's instructions given on the emissions decal.
- The idle speed adjustment procedure varies with each vehicle, engine, or model year.
- The ignition timing should be checked and adjusted as necessary prior to the idle speed check.
- With the transmission in neutral and the parking brake applied, turn all the accessories and lights off.
- Make sure the choke plate is open and the throttle linkage is off fast idle. Also be sure the engine is at normal operating temperature.
- Connect a tachometer to the coil negative primary terminal and ground.
- Disconnect the cooling fan motor connector, and connect 12 volts to the motor terminal so that the fan runs continually.
- Disconnect all items recommended in the vehicle manufacturer's service manual. In some cases, this may include disconnecting and plugging the vacuum hoses at the exhaust gas recirculation (EGR) valve and air cleaner heated air temperature sensor.
- Other manufacturers recommend removing the positive crankcase ventilation (PCV) valve, and allowing the valve to pull in underhood air.
- Vehicles with feedback carburetion might also require that certain connectors be disconnected to keep the carburetor in open loop.
- Observe the engine's speed. If the idle speed is not correct, adjust the speed using the screw on the kicker solenoid or the idle-speed adjusting screw.
Idle speed screw on a solenoid throttle kicker. Courtesy of DaimlerChrysler Corporation.
- Then reconnect the feedback connector, PCV valve, and kicker solenoid connector.
- Increase the engine rpm to 2,500 for 15 seconds, then allow the engine to idle.
- If the idle speed changes slightly, this is normal and a readjustment is not required. However, if the idle speed changes greatly, that may indicate another problem.
- If the idle speed adjustment is okay, disconnect the jumper wire and reconnect the fan motor connector.
Fast Idle Speed Check
Instructions for setting fast idle speed are also contained on the emissions decal or in the manufacturer's service manual.
- Most carburetors have a fast idle screw that can be adjusted to correct the fast idle speed.
- After satisfying any pretest conditions (such as A/C off or transmission in gear), place the specified step of the fast idle cam on the adjusting screw.
- Turn the screw clockwise to increase rpm and counterclockwise to decrease rpm.
- Place the fast idle screw on the second highest step of the fast idle cam, against the shoulder of the highest step.
- Hold downward lightly on the fast idle control lever, and measure the specified clearance between the lower edge of the choke valve and the air horn wall.
- If an adjustment is necessary, bend the fast idle connector rod at the lower bend.
Adjusting the fast idle cam by bending tang on the choke shaft. Courtesy of DaimlerChrysler Corporation.
- If the carburetor has a vacuum/electric throttle kicker, fast idle speed is checked and adjusted by operating the air conditioning system.
- Idle speed should increase when the A/C compressor is turned on. The throttle kicker is activated when the driver selects the A/C mode and when the ignition switch is turned on.
- Check the service manual for any specifications that may be given for idle speed with the A/C on.
- Prior to any idle speed check, the engine must be at normal operating temperature.
Idle Mixture Adjustment
As stated earlier, idle mixture adjustments have been eliminated on the newer carburetors.
- On older carburetors that allow for idle mixture adjustment, make sure the idle speed is set to the specified rpm.
- Turn the mixture screws to obtain the smoothest idle.
- Then readjust the idle speed to specifications.
- Repeat until the engine idles smoothly at the correct engine rpm.
- Turn the idle mixture adjustment screw in as far as possible without a loss of smoothness.
- Some procedures require leaning the mixture until there is a definite drop in rpm and loss of smoothness and then backing out the idle mixture screws one quarter.