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.
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.
- 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.
- Connect a fuel flow meter.
- Start the engine and set the approximate idle speed with the idle air screw.
- Adjust the idle fuel flow to specifications with the idle fuel screw.
- Use the idle air screw to set the idle speed again.
- Repeat this procedure (Steps 4 and 5) until both the idle fuel flow and
the idle speed are within specifications.
- Disconnect the fuel flow meter.
- If you have not already done so, adjust the throttle angle opening to specifications.
Make the adjustment from the fully closed position.
- Lock the nut after adjustment.
- Start the engine and adjust the idle speed with the idle air screw.
- 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.
- 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.
- Recheck the idle speed and adjust if necessary, using the idle air screw.
- 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.
