Idle Mixture Adjustment

PISTON ENGINES

All 1978–82 Models; 1983 GLC Wagon and 1986–87 Carbureted 323

Idle mixture adjustment for all 1978–82 Mazda piston engine cars, as well as 1983 GLC Wagons and 1986–87 carburetor-equipped 323s, requires the use of an HC/CO analyzer. Since this is an extremely expensive piece of equipment, the adjustment procedure is complex, and this adjustment is not required as a part of ordinary maintenance, no procedures are included here. This adjustment can, however, be checked at a reasonable cost at many diagnostic centers, or may be checked during mandatory vehicle emission inspections, where applicable. If the engine exhibits a rough idle and/or smoke, combined (in some cases) with hesitation or poor running at low speeds, the idle mixture might be at fault. First, check all the basic tune-up adjustments such as dwell, ignition timing, and spark plug condition, and verify that there are no vacuum leaks due to disconnected or leaking hoses. Then, if these symptoms persist, have the idle mixture checked with an HC/CO meter at a dealer or diagnostic center.

NOTE: If incorrect readings are detected, we suggest you take the vehicle to a shop or dealer familiar with Mazdas and have the idle mixture adjusted according to the procedure specified by the factory.

1983–85 GLC (Except Wagon) and 626

NOTE: The carburetor must be removed and disassembled to gain access to the mixture screw since an idle mixture adjustment is normally only necessary in the event of carburetor overhaul. For information on carburetor removal and installation, please refer to Section 5 of this manual.

The air/fuel mixture for most 1983–85 carbureted piston engine models can readily be checked with a dwell/tachometer. An incorrect reading can indicate either an improper idle mixture or a faulty oxygen sensor. Make sure that the engine is properly tuned and that all vacuum hoses are properly connected before suspecting an improper idle mixture.

NOTE: Do not secure the spring pin (which locks the mixture adjusting screw) until the adjustment has been performed.

To check/adjust the idle mixture:

1. On 1984 626s only, disconnect and plug the idle compensator, thermosensor, and reed valve hoses.
2. Connect a tachometer to the engine.
3. Connect a dwell meter that can register up to 90 degrees (and which is suitable for four-cylinder engines). Connect the meter's positive (+/red) lead to terminal Y of the Air/Fuel (A/F) solenoid valve service connector, and the negative (−/black) lead to the engine.

   Fig. 1: The mixture adjusting screw can be turned with a screwdriver

   Fig. 2: A spring pin is used to lock the mixture adjusting screw in place

   Fig. 3: Gently tap the spring pin into position after adjusting the idle mixture

4. Warm up the engine to normal operating temperature and turn off all accessories.
5. Make sure the idle speed meets the following specifications; adjust it (if necessary), as described earlier in this section:
   • 1983–85 manual transaxle GLC: 850 rpm in Neutral
   • 1983–85 automatic transaxle GLC: 1,050 rpm in N (Neutral)
   • 1983–85 manual transaxle 626: 750 rpm in Neutral
   • 1983–85 automatic transaxle 626: 700 rpm in D (Drive)
6. With the engine running at the designated idle speed, note the reading on the dwell meter; the reading should be 32–40 degrees.
7. If the dwell reading is not within the appropriate range, adjust the idle mixture by turning the mixture adjusting screw, located near the base of the carburetor.
8. Make sure that the idle speed still meets specifications.

   NOTE: Idle speed of the GLC with automatic transaxle should now be checked with the gear selector in D (Drive), but MAKE SURE the parking brake is set and the drive wheels are blocked. The correct reading is 750 rpm.

9. After the mixture is properly set, press in the spring pin to lock the adjusting screw.
10. On 1984 626s only, unplug and reconnect the idle compensator, thermosensor, and reed valve hoses.
11. Disconnect the dwell meter and tachometer. Re-attach the A/F solenoid valve connector.

1986–89 Fuel Injected Models

On all 1986–89 fuel injected piston engine vehicles covered by this manual, idle mixture is automatically maintained by the electronic engine control unit, and thus, cannot be manually adjusted.