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How to adjust the mixture screws on your carb

How to adjust the mixture screws on your carb

This is an easy one as far as the basics go. 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 or whatever they were told. That would be great if it was that easy but not every engine is the same so not every carb will need the exact same adjustment.

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 mixture screw settings.

Luckily, adjust them is almost that easy and here's how you do it. Start the engine and let it warm up. Set the idle screw (the one with the spring under it on the actual throttle linkage) so that the engine is idling at a normal RPM.

On a 4 barrel, start with the primary side of the carb (the front half) and take a small screwdriver and start turning one bleeder screw on one side of the carb inward (clockwise) until the idle either drops or rises. If it drops, you are going the wrong way, so stop and 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, it's because the engine liked what you just did, so now go back and re-adjust the actual idle speed screw on the linkage to bring the idle back down to the normal RPM again and then repeat what you just did to the other side of the carb. When you are done with that side, and you found the highest possible idle, now go back and re-adjust the idle speed screw again so that the engine is idling back where it should be idling at and you're done.

If your carb has what's called "4 corner idle circuits", like many modern performance carbs have, then just repeat what you just did to the front ones except on the rear ones this time. A word of note though; often times the rears aren't really necessary, so in some, if not "many" cases, the rear ones either don't make much difference, or sometimes you can just screw them in all the way and then crack them open about a half a turn so that most of what is supporting the engine's idle mixture is being supported by the primary (front) mixture screws. On alot of 4 barrel carbs, there only mixture screw on the primaries.

Carbs like a Edelbrock 4 barrel only have 2 mixture screws in the front, so you adjust those like you would how I explained by screwing one in until the idle drops and then back out again until you get the highest possible idle, then go to the other screw and do the same thing. 

Keep in mind, these are IDLE bleeds or "idle mixture screw" 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 wrong. The only thing you are adjusting with these bleed screws is the IDLE mixture circuit. 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 of fuel from the accelerator pump, then sometimes cracking-open (counter clockwise) 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 is by changing the main jets which are inside the carb and aren't adjustable from the outside unless you have a customized carb with a "Dial-a-Jet" system, on it, which are pretty rare, otherwise you have to open-up the carb and swap the main jets out to either a larger size to richen it, or a smaller size to lean it it. But in retrospect, changing the main jets has no affect on the idle either. They both do separate things. The idle mixture screw only affect the idle, and the main jets only affect driving down the road or the track.

How to adjust your carburetor

This is the idle speed screw on a Holley. They always have a spring underneath them to help hold them in place. On almost any brand or type of carburetor, they are located on the throttle linkage and almost always have a spring underneath the head. This screw only adjusts the idle RPM. It has nothing to do with the mixture. 

How to adjust your carburetor

Now, there are other adjustments that can be done to also modify (richen or lean) the idle. One way is by the slow speed air bleeds. In the older days, to modify these we had to drill them out a few thousandths of an inch with a numbered drill to allow more air to be pulled into the idle circuits inside the carb and then adjust the richness again with the mixture screws, but on more modern performance carbs, they are all screw-in and interchangeable and drilling them is no longer necessary. On top of that, on "most" engines, changing these isn't usually necessary at all.

How to adjust your carburetor

You can also adjust the secondary throttle plates with a screw that's usually hidden on the underside of the base plate on the passenger side of the carb, but this is usually only in extreme cases of idling issues, like when your cam is way too big, or you don't have a slippery enough stall converter and it doesn't want to idle in gear, these other adjustments "can" help solve an issue like that if it's stalling or something, but for basics, we're just explaining the idle mixture screws that the average guy (or gal) would normally adjust, and now you know how :-)  

How to adjust your carburetor

Here is that screw on the underside of the throttle plate that adjusts the secondaries. 


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