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@ AIR MIXTURE SCREW ATC90_ALL CT90_1968_TO_77 CT70_1969_TO_77 (217X)



@ The air screw adjusts the amount of air that passes over the opening The air screw adjusts the amount of air that passes over the opening of the slow jet, using vacuum to pull fuel up from the bowl. Since the throttle slide is almost completely closed at idle, the air screw and slow jet bypass the closed carb venturi and act as the baseline providers of fuel mixture for idle and low speed throttle response. You add a little fuel and air when you set the idle screw up a bit, but without the slow jet throttle response in the low registers would be slow and choppy.

The air screw should have an effect on the idle speed - just not as much as the idle screw.

If you have a limiter cap on the screw that won't let it turn more than one time - it shouldn't be too far off the right setting .

Ideally, with the cap removed or the tab on it cut off (we use diagonal cutters to remove it), you would dial the air screw all the way in clockwise until it stops. Then you use the typical starting setting of 2-1/2 turns out counterclockwise (900 degrees).

Start and warm up the engine on choke, then turn the choke off.

With the engine idling a little fast (turn the idle screw in clockwise a little), turn the air screw in and out until the idle speed rises and falls. Find the fastest idle and the air screw is set properly.

Dial the idle back down to a reasonable rpm, and you're done.

If the idle does not change with changes in the air screw setting, there is a clog in the slow jet or the air passages inside the carb related to the jet and the air screw.

It's then time to remove the carb, disassemble it and give it a thorough cleaning in chlorinated solvent. Blow all the internal passages clear with compressed air, reassemble and give it another try.


If you turn it clockwise it will eventually bottom out in the "seat" that the needle rests in. Turn it inwards gently to avoid damaging this part. Then turn it counterclockwise out 2-2 1/2 turns.

Also many times the air screw has almost no affect because the air passage OR the slow jet are plugged up. __________________________________________________

Mike G.