Adjusting Idle Fuel Mixture

Q: Adjusting Idle Fuel Mixture

The idle fuel mixture seems to be too rich. I have fiddled with the two idle screws on the bottom of the Q-Jet carb but it still runs too fat. Stinks up the neighborhood. Is there a systomatic way of trying to lean out the idle rather than me just turning screws.

A: The mixture screws are the only control for idle fuel mixture.The carb’s main jets control the mixture the rest off the time.The idle circuit is metered through the main jets,then through some calibrated restrictions which are difficult to modify. Changing the main jets will have no measurable effect on the idle mixture.I have found though,that sometimes people think the mixture is too rich when they smell unburned fuel coming out the exhaust and the problem is actually that it’s too lean causing poor idling/misfiring and the unburned fuel smell is from the misfiring.Obviously you want to check for vacuum leaks,timing,dwell (if you have points) etc.Lastly,a high mileage engine may emit a lot of unburned fuel from lack of cylinder sealing even though it seems to run well.Just some food for thought.

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