The air/fuel mixture is measured by sampling the exhaust with a CO meter. If this equipment is not available, no checking or adjustment is possible. Since adjusting the mixture is not part of a normal tune-up, the adjusting screw is protected with a tamper-proof plug. If the air/fuel ratio is obviously incorrect (lean miss, sooty spark plugs and tail pipe), or if the CO measurement is out of specification, look for other problems such as a vacuum leak, bad sensors, loose electrical connections, and correct any faults before deciding to adjusting the mixture.
- With the engine at normal operating temperature, make sure the ignition timing and idle speed are correct. Stop the engine.
- Disconnect the crank case breather hose and move it so only fresh air can enter.
- Insert the CO meter probe into the CO test point at the back of the engine compartment, a metal tube with a plastic cap. Make sure the probe is a tight fit and will not draw in outside air.
- Start the engine and turn OFF all lights and accessories. If any fuel injection lines have been disconnected, rev the engine to 3000 rpm, 2-3 times to clear any air from the system.
- The CO reading should be between 0.2-1.2 percent. Stop the engine.
If the CO reading is not correct, remove the mixture adjusting screw plug as follows:
- Center-punch the mixture adjusting screw plug.
- Drill a 3 / 32 in. (2.5mm) hole approximately 5 / 32 in. (4mm) deep in the center of the plug. It is helpful to apply a small amount of grease to the drill bit to catch the metal shavings.
- Thread a 1 / 8 in. (3mm) sheet metal screw into the hole.
- Remove the plug using a pair of pliers.
- Start the engine and run it at idle, then insert tool P377 or a 3mm Allen wrench into the hole. Adjust the screw slightly (clockwise increases the CO reading), then remove the adjusting tool. Briefly accelerate the engine and check the CO reading again. Continue adjusting until the CO reading is within specification.
- Drive a new mixture adjusting screw plug into the hole.