carb adjustment - idle mixture of throttle adjusting as the final step to set RPMs?

Feb 28, 2022
Austin, TX
I can't seem to find a definitive answer. Assume we're going by the book. For the final idle setting back down to 650 (1973 fj40) should I use the idle mixture screw of or the idle setting screw? My book doesn't clarify and I see references to people using both as the last step.

Below is my summary from the manual I have:
Bring to operating temp
  1. Connect Tach
  2. Connect Vacuum Gauge
  3. Throttle adjusting screw (idle speed adjustments) to lowest possible where engine runs
Repeat a few times
  1. Idle mixture in/out to obtain highest steady vacuum at idle speed
  2. Throttle adjusting screw (idle speed adjustments) to highest vacuum reading
Final Step
  1. Set engine idle speed (which screw?)
Thanks -- Dave


Mar 1, 2015
Oroville, CA
There is a good Youtube on the lean-drop on a 2f which is what I believe you are describing, should come up top in a search and is helpful. The way I read it, the idle adjuster is the last step, but in real life it kinda ends up being whatever works best, not exact. Do it slow, it takes time for the engine response usually.

Your highest vacuum is going to be highest rpms, so using the lean-drop you raise RPMs with mixture screw, then lower RPMS with idle speed screw back and forth until your happy.
Sep 20, 2022
35th parallel north
Step 1: check dwell and spark timing
Step 2: visually check spark plugs for anything black, (consider using F-motor-spec on the plug gap until you think that your out of the woods for carbon build-up)
Step 3: check vacuum, if above factory spec, set idle to about 600 or less with idle speed screw, and add richness to the mix to smooth it out. If it is set too lean, it wastes fuel so don't worry about opening the mix screw too much here, other than it isn't as good of a transition to part-throttle.
Step 4: check plugs after a few tanks of gas for carbon or oil

At some point, you can set your valve lash tighter than factory spec to account for any grooving on the rocker arms, it might take away an audible click away in the process.
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