Ford Mid-Size Cars 1971-1985 Repair Guide

Idle Mixture Adjustment


See Figures 1, 2 and 3

For this procedure, Ford recommends a propane enrichment procedure. This requires special equipment which is often not available to the general public. In lieu of this equipment the following procedure may be followed to obtain a satisfactory idle mixture.

  1. Block the wheels, set the parking brake.
  3. Connect a tachometer according to the tool manufacturer's instructions.
  5. Run the engine to bring it to normal operating temperature.
  7. Disconnect the hose between the emission canister and the air cleaner.
  9. On engines equipped with the Thermactor® air injection system, the routing of the vacuum lines connected to the dump valve will have to be temporarily changed. Mark them for proper reconnection before switching them.
  11. For valves with one or two vacuum lines at the side, disconnect and plug the lines.
  13. For valves with one vacuum line at the top, check the line to see if it is connected to the intake manifold, or an intake manifold source such as the carburetor or distributor vacuum line. If not, remove and plug the line at the dump valve and connect a temporary length of vacuum hose from the dump valve fitting to a source of the intake manifold vacuum.
  15. Remove the limiter caps from the mixture screws by CAREFULLY cutting them with a sharp knife.
  17. Place the transmission in N or P and run the engine at 2500 rpm for 15 seconds.
  19. Adjust the idle speed to the higher of the two figures given on the underhood sticker.
  21. Turn the idle mixture screws to obtain the desired rpm, leaving the screws in the leanest position that will maintain this rpm.
  23. Repeat steps 9-11 until further adjustment of the mixture screws does not increase the rpm.
  25. Turn the screws in until the lower of the two idle speed figures is reached. Turn the screws in 1 / 4 turn increments to insure a balance.
  27. Turn the engine OFF and remove the tachometer. Reinstall all equipment.

Rough idle, that cannot be corrected by normal service procedures on 1977 and later models, may be caused by a leakage between the EGR valve body and diaphragm. To determine if this is the cause:

Tighten the EGR bolts to 15 ft. lbs. (20 Nm). Connect a vacuum gauge to the intake manifold. Lift up on the diaphragm to exert a sideways pressure on the diaphragm housing. If the idle changes or the reading on the vacuum gauge varies, replace the EGR valve.

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Fig. Fig. 1: 5200/6500 2-V carburetor with adjustment positions

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Fig. Fig. 2: On later model 2150 carburetors, the metal plug cap must be carefully removed before any adjustments are made.

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Fig. Fig. 3: 7200/2700 carburetor with curd idle adjustment