Here we will try to give you ideas on how to adjust a carburetor. This should only be done after the engine has been checked for any mechanical, vacuum, or electrical problems that may effect the way the engine runs. REMEMBER, ONLY AFTER CHECKING FOR MECHANICAL, VACUUM, OR ELECTRICAL PROBLEMS FIRST! The transmission should be in neutral and apply the parking brake. All accessories should be turned off. Refer to the under-hood sticker, then check the ignition timing.

Adjustments must be done after all of the electrical connections, vacuum hoses, and lines are hooked up. The PCV valve, Gulp valve, Diverter valve, & Altitude compensator must not be leaking, and there must be no other vacuum leaks. Air conditioning must be turned off. Vacuum leaks are common, and they must all be fixed first. On Japanese vehicles (Datsun, Toyota, Honda, etc.), it is common to find a bad grounding circuit up to the carb or no power to one or more of the solenoids or to the choke. Be careful to check for this and fix it first. Remember, you will never get your carb adjusted right if your engine has mechanical problems, vacuum leaks, or needs a tune up.

Doing it

First, you should know that it is common for carbs to flood (dump gas) when they are first installed. It’s caused by the big rush of gas into the empty carb which often carries along very tiny particles of dirt, rust, etc, which get lodged under the needle (of the needle & seat assembly). If this happens, be aware this is not a “warranty situation”. Get it fixed.

To begin the procedure, start the motor and allow it to warm up to operating temperature (15-20 minutes). Make sure that the choke is all the way open and the fast idle cam is no longer engaged. FIRST, set the idle speed according to specs; LASTLY, adjust the mixture. The general idea is to get the mixture set as lean as possible, but not to where the RPM is dropping off a lot, and without affecting engine performance. This will usually be just as the RPM begins to drop on the lean side of best running. It will probably run OK if you set it richer, but will just use more gas and probably won’t pass the smog test. Always start with the mixture a little too rich and adjust towards lean (turning it in).

Note that most mixture screws turn inward (clockwise) to make lean, and out for rich, although there are a few Holley 4bbl carbs that you turn out to make lean. These screws control the fuel being delivered to the engine at idle only. Adjusting them will not affect how much gas the car uses except at idle. If there are 2 screws on the carb, they are both gas control screws and must be adjusted equally. Don’t have one set at 2 turns out and the other at 3 turns out. If you turn one a little, also turn the other the same amount and same direction. REMEMBER, SET THE MIXTURE AS THE LAST ADJUSTMENT. DON’T GO BACK & PLAY WITH THE IDLE SPEED AFTER SETTING THE MIXTURE, BECAUSE CHANGING THE IDLE SPEED WILL GREATLY AFFECT THE MIXTURE. If you did play with the speed, you will need to reset the mixture again.

When you turn the mixture screw, turn it slowly, no more than 1/2 turn every few seconds. It takes 2 to 5 seconds for the gas to trickle through the carb and actually have an effect on the RPM of the engine. If the engine has been idling for an extended period of time, blip the throttle a couple of times to “normalize” the idle speed. Let it settle back to idle, then continue with your mixture adjustment.

If your car has a “throttle positioner” or “throttle position sensor”, you may need to adjust that. See your manual. You may also need to adjust the “fast idle adjustment” if the engine ran too fast or too slow when it was still cold. It should run faster when cold but within factory recommendations. Refer to underhood sticker.

Remember that adjusting the mixture is always the last adjustment that is made. Don’t use the mixture screw to adjust the idle speed. If you decrease the idle SPEED, the mixture will get leaner and you may have to enrichen, and vice versa. Don’t go by your repair manual’s recommendation of setting it at “2 turns out or 3 turns out”. This is a bench setting only, not the final setting and it will likely not run well at all if you leave it there. We have already preset the mixture much closer to correct than that. You won’t need to turn things much at all.

If you turn the idle speed screw way up (faster), you will by-pass the idle circuit, so then turning the mixture screw will then have little or no effect on idle. If the idle speed is set too fast, you will be too rich at idle, “dumping fuel”, and the car will not pass smog (because you are trying to idle on the progression circuit – believe me, it won’t work.) This is one of the MOST common cause of “defect” returns to us. Be careful, do it right.

If you have trouble passing smog at idle, reset the idle speed slower and then readjust the mixture, first going out rich and then slowly turning in leaner until you find the lean drop point; it will probably pass then.

If this procedure didn’t work for you, then go back to the beginning and check for mechanical, vacuum, and electrical problems, like you should have at the beginning.