Mazda Trucks 1972-1986 Repair Guide

Idle Mixture

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GASOLINE ENGINES



If for some reason (tampering or carburetor overhaul) the mixture needs adjustment, use the following procedures. Two procedures are given; one for a HC/CO analyzer and one for a fuel flow meter.

FUEL FLOW METER

It is best to use this procedure to set the idle and throttle screws after they have been disturbed. After you are finished, check the adjustment with a HC/CO analyzer.

  1. Adjust the throttle angle opening to specifications with the throttle adjustment screw. The adjustment should be made from the fully closed position. Tighten the locknut after the adjustment is complete.
  2.  
  3. Connect a fuel flow meter.
  4.  
  5. Start the engine and set the approximate idle speed with the idle air screw.
  6.  
  7. Adjust the idle fuel flow to specifications with the idle fuel screw.
  8.  
  9. Use the idle air screw to set the idle speed again.
  10.  
  11. Repeat this procedure (Steps 4 and 5) until both the idle fuel flow and the idle speed are within specifications.
  12.  
  13. Disconnect the fuel flow meter.
  14.  

HC/CO ANALYZER
  1. If you have not already done so, adjust the throttle angle opening to specifications. Make the adjustment from the fully closed position.
  2.  
  3. Lock the nut after adjustment.
  4.  
  5. Start the engine and adjust the idle speed with the idle air screw.
  6.  
  7. Using the gas analyzer, check the HC (hydrocarbon) and CO (carbon monoxide) readings. If the HC is less than 200 ppm (parts per million) and the CO is between 0.1-2.0%, no further adjustment is needed.
  8.  
  9. If the HC and CO are not within specifications, adjust the CO reading to as close to 0.1% as possible, keeping the HC reading below 200 ppm. Use the idle fuel screw to make this adjustment.
  10.  
  11. Recheck the idle speed and adjust if necessary, using the idle air screw.
  12.  
  13. Recheck the HC and CO readings to be sure that they are within limits. Repeat Steps 5 and 6 until the CH, CO and idle speed are all within specifications.
  14.  

 
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