Nissan Sentra/Pulsar/NX 1982-1996 Repair Guide

Idle Mixture Adjustment



See Figures 1 and 2

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Fig. Fig. 1: Drill out the seal plug which covers the mixture adjusting screw

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Fig. Fig. 2: Install a new plug so that its top is slightly below the surface of the bore

Idle mixture is adjusted on carbureted vehicles only, and then only by using a "CO meter'', an expensive engine testing device used typically by well-equipped repair shops in the repair of emission system problems. Before attempting to adjust CO, make sure there are no vacuum system leaks, tuning problems (idle speed, ignition timing, spark plug condition or gap problems), or engine operating problems such as burned or misadjusted valves. It is possible that, if your car idles roughly in the absence of any of these problems, the cost of having the CO adjusted can be reduced by following the procedure given here.

  1. Remove the carburetor from the engine as described in Fuel System .
  3. Locate the seal plug to the right of and below the idle speed screw. Select a drill bit that is considerably smaller than the diameter of the orifice in the carburetor casting into which the plug is mounted.
  5. Drill very cautiously and slowly. Feel for the point where the drill just penetrates the inner end of the plug. Don't drill farther, or the carburetor mixture screw will be damaged.
  7. Once you have drilled through the plug, use a less brittle metal object that will fit through the hole in the plug to pry it out of the carburetor orifice. Carefully clean all metal shavings out of the bore so the mixture screw threads will not be damaged.
  9. Reinstall the carburetor. Be sure to use a new gasket and make sure all vacuum lines are securely connected.
  11. Have the person adjusting the CO with the required equipment follow these procedures:
    1. Run the engine until it is hot. Then, shut it off.
    3. Disconnect the air/fuel ratio solenoid harness connector. Disconnect the air induction hose from the air cleaner and plug or cap the hose.
    5. Start the engine and race it three times to 2,500 rpm. Then, allow it to idle.
    7. Insert the CO meter probe 16 in. (41cm) into the tailpipe. Read the CO level. If necessary, adjust the mixture screw to obtain 4-6% for California and Canadian cars, and 2-4% for 49 States cars.

  13. Tap a new seal plug squarely into the carburetor bore with an object with a perfectly flat front. The object should be just slightly smaller than the bore.
  15. Restore all disconnected hoses and electrical connectors. Then, repeat the idle speed adjustment procedure above.


Fuel injected engines fuel mixture is controlled by the Electronic Control Unit (ECU). No fuel mixture adjustment is necessary or possible.