Proper idle air/fuel adjustment

[Bills64]

Confused on how to properly adjust idle air/fuel adjustment without a a/f gauge. From what I've read, adjust curb idle to lowest rpm as possible then start adjusting one side til you get the highest rpm then the other side til that is the highest. After that then adjust the curb idle to the specified idle. This is all a question. Can't really use a vacuum gauge because of a tall cam. Any guidance would be greatly appreciated.

[77cruiser]

Highest rpm or highest vacuum would be the same.

[Bills64]

Thank you

[Cliff Ruggles]

An A/F gauge is useless for setting the idle mixture screws. 

Assuming your carb has adequate idle fuel to the mixture screws and bypass air so no "nozzle drip" get the engine FULLY warmed up and well heat soaked.

Check the timing to make sure it's where it's supposed to be.

Set the curb idle speed.

Make sure the mixture screws are backed out sufficiently so the engine is a bit rich at idle.  Slowly move them in one at a time and listen for a speed change or RPM drop in the engine.   Stop, then back that screw out until the engine just speeds up and runs smooth.  Move to the other screw and do the same thing.  If the intake is fully divided you are pretty much done, if open under the carb you may need to balance the screws some. 

The goal here is to obtain the best idle quality at the leanest settings.  Once you have done this take a gander at your A/F gauge, this will be your base settings.....

[novadude]

What Cliff said.  Just a few additions...

I start with a baseline setting of like 5 turns out on the late model fine-pitch metric screws.  Make sure they are both out the same amount of turns from seated to start.  I alternate screws while watching the hand-held tach, and dial them in 1/4 turn at a time, and watching for the RPM drop.  This ensures that they are both screwed in/out an equal amount of turns.

I think that is what Cliff was saying, but it might be confusing to some that read it (the "alternate screws" part).  IMO you don't want to just keep turning one screw in until the rpm drops.  Good idea to keep them balanced.

[Bills64]

Thanks to all!

[Cliff Ruggles]

For the most part 1979-up carbs with the metric thread pitch mixture screws are 2 to 1 for turns to adjust if you are counting.  So most later carbs come in around 5-7 turns vs 2.5-3.5, for example.  Also be aware that the factory also downsized the holes under the mixture screws, plus put tamper proof covers over them.  This was done so even if you removed the covers you were NOT able to add a lot of fuel with the mixture screws because the restriction under them was too small.......

[lightning boy]

You'll be able to hear the exhaust sound change when you tighten the mixture screw down. I call it a lean roll.