Slowly turn each idle mixture screw clockwise in and count the number of turns until it lightly seats. Generally this will be anywhere from 1 to 2 turns out from fully seated. When seating these screws, do so gently to prevent damaging the seats in the metering block. Do this for both or all four idle mixture screws and set them to the same position.
With the engine idling in Park with the emergency brake engaged, the first step is to set the idle speed at the desired rpm.
The curb idle is set with the idle speed adjustment screw on the primary throttle linkage on the driver side of the carburetor. Repeat this process and evaluate the result. This first adjustment sequence is to place the idle mixture screws on both sides of the carburetor to achieve the highest idle vacuum setting possible. If after an initial adjustment, the idle speed increases beyond the desired rpm, turn the idle speed screw counterclockwise to return the speed to the original engine speed.
This will probably lower the idle vacuum reading, which is normal. Do this on both sides of the carburetor and evaluate the result.
If the vacuum drops, then return to the highest setting and see if the vacuum reading increases. This means you are close to your final setting. Continue to make very slight changes to the idle mixture screws on both sides. The idle speed screw arrow 1 is located on the primary linkage. The idle metering adjustment screws arrow 2 are located on the primary metering block on most Holley carburetors. These adjustments will not be more than literally the width of the slot in the adjustment screw.
These are very small changes but do have an effect on idle quality. Moving back and forth adjusting both idle mixture screws the same for each change will eventually create the highest idle vacuum level.
This should be achieved with the idle mixture screws adjusted with an emphasis toward lean settings. If turning the idle mixture screws out richer does not improve idle vacuum or engine speed, then the leaner setting is always the preferred choice. At this point, have a friend sit behind the wheel and place the transmission in Drive with his foot firmly on the brakes. Monitor the vacuum gauge and make sure the idle is stable.
Placing the engine in gear will produce a slightly lower vacuum gauge reading. This is normal because a load has been applied to the engine. Place the transmission back into Park and allow the idle rpm to stabilize. The engine should now idle at the desired idle speed with the highest manifold vacuum.
Normally, this is where most adjustment recommendations end. But for an ideal idle mixture setting, a slightly leaner idle mixture is beneficial - roughly the equivalent width of the idle mixture screw slot. What this does is make the idle mixture slightly leaner which reduces excess hydrocarbons unburned gasoline in the exhaust.
This will tune your engine to idle a tiny bit leaner. If the idle is unacceptable, return the idle mixture screws to the highest vacuum setting, but in most cases this slight lean setting will still support quality idle performance. This is especially true if the engine is coupled with a somewhat tight torque converter.
A richer idle mixture may be required in order for the engine to idle against a tight converter. Of course, this is not an ideal situation. The ideal solution is to couple this engine with a looser torque converter to reduce the load.
Another lean idle situation that can occur is a slight off-idle hesitation. In certain cases, this may not be cured with an increased accelerator pump shot.
One solution might be to try a slightly richer idle mixture. It's an unfortunate but fairly common situation where street engines with long-duration camshafts with significant overlap will exhibit this off-idle hesitation and often the only way to cure it is with a richer idle mixture.
What this process will do is produce a quality idle mixture setting that will allow your engine to run sweet, clean, and true. You can save the Superman costume for later technical adventures. Most street Holley carburetors position the manifold vacuum port in this location arrow on the carburetor mounting base. Remove the plug and attach the vacuum gauge here. With the idle mixture screw removed, the tapered portion is what adjusts the volume of fuel allowed into the engine.
Turning this adjustment screw clockwise in reduces the amount of fuel. Turning the screw counter-clockwise out , increases the amount of fuel delivered to the engine. The idle mixture screw A area pulls fuel from the idle feed restrictor located on this metering block at arrow B. Many Holley performance carburetors are equipped with what is called four-hole or four-port idle mixture control. These carbs employ two more idle mixture screws on the secondary side of the carburetor.
The adjustment procedure is exactly the same except that all four idle mixture screws are adjusted instead of just two. Turning the idle mixture screws counter clockwise out richens the mixture. For Rochester Quadrajets pick up an idle mixture adjusting tool at most auto parts stores.
The tool bends which will make adjusting easier. Inspect the tip and thread carefully for grooves. Replace any screw that shows signs of wear or damage. Make sure the choke valve is completely open. You may have to rev the engine slightly so that the fast idle cam moves to the idle position. Adjust the idle to specification. Using a vacuum meter Hook the vacuum meter to one of the vacuum ports on the intake, or the carburetor. Take turns with each idle mixture screw. Turn each screw out a bit for a start maybe 1 turn.
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