This is an easy one. Some people think you're supposed to turn the idle mixture screws all the way in, and then back them out 2 1/2 turns. That would be great if it was that easy. The fact of the matter is; the idle mixture screws are directly affected by the engine's displacement, the cam profile, idle vacuum, timing, etc., and being that pretty much any carb can be put on any engine, how could a "standard" 2 1/2 turn adjustment possibly work on every engine? It can't. You can't put a 750 carb on a 302, and then the same identical carb on a 540 cubic inch big block and expect to have the same idle bleed settings.
Here's how you adjust them. Start the engine and let it warm up. Set the idle screw so that the engine is idling at a normal RPM. Take a small screwdriver and start turning one bleeder screw on one side of the carb inward until the idle either drops or rises. If it drops, you are going the wrong way, so turn it the other way until the you hear the engine's idle rise back up again. If it raises, keep turning it SLOWLY until it peaks-out and starts to drop again. So what you want to do is adjust the screw until you get the highest idle. If the idle went up because the engine liked what you just did, go back and re-adjust the actual idle speed screw to bring the idle back down to the normal RPM again, and repeat what you just did to the other side of the carb. When you are done with that side, and you have found the highest possible idle, so go back again and re-adjust the idle speed screw so the engine is idling back where it should be idling at.
Keep in mind, these are IDLE bleeds only. In other words, the second you crack-open your throttle, these idle bleeds quit working. Some people think they can lean out or richen a carb by simply adjusting these screws, and that is SO wrong. The only thing you are adjusting with these bleed screws is the IDLE mixture. Yes, they DO affect an "off-idle" mixture a tiny bit, so if your engine has a slight stumble right when you start to crack-open the throttle and you know you are getting an ample squirt from the accelerator pump, then sometimes cracking-open the idle mixture screws another quarter to half turn "can" make that stumble go away, but not in all incidences. The ONLY way you can "really" lean-out or richen-up a carb from the outside is if you have a customized carb with a "Dial-a-Jet" system, on it, otherwise you have to open-up the carb and swap the main jets out to either a larger or smaller size.
If you want to learn more on how to rebuild, modify, and tune carbs, in great detail, be sure to check out our "Performance Carburetors" DVD set on our Auto Shop Videos page by clicking HERE.
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