Idle Hands

How to Set Idle Mixture Correctly

By Jeff Smith   –   Photography by the Author

The Bible says that idle hands are the devil’s workshop. Our take is a little different. When it comes to setting the idle mixture on your street-driven engine, your hands will be plenty busy adjusting, tweaking, and setting several different parameters. This may not sound like something to account for an hour or so of your time, but trust us, it’s worth the effort.

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If you think about it, a street-driven engine spends much of its time running with a closed or near-closed throttle. Waiting for the engine to warm up, sitting at a stoplight, cruising down the highway, or on city streets are all conditions where the throttle is just barely open. With most carburetors, these situations require a throttle opening of roughly 10 to 15 percent. This places throttle action well within the realm of the idle circuit.

001 idle mixture adjustment screws on a modular Holley four barrel are found on the sides of the primary metering block
The idle mixture adjustment screws on a modular Holley four-barrel are found on the sides of the primary metering block. Most Holley four-barrels only use idle mixture screws on the primary side, but more and more performance carburetors use what is called a four-hole idle with mixture screws on the secondary side as well.

That statement may come as a bit of a surprise to many automotive enthusiasts. But if your street engine idles at roughly 12 inches of manifold vacuum or higher, then your engine probably runs down the freeway at not much more than 15 percent throttle opening, which falls within the control area of the idle circuit in your carburetor.

Conversely, if your engine has a big cam and idles at less than 10 inches of manifold vacuum, then the engine probably runs down the freeway (based on several variables we won’t bother to list here) on the primary main jet circuit. This story will concentrate on the milder of these two options.

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002 GM Quadrajet places the idle mixture adjustment screws at the base of the throttle plate in the front
The GM Quadrajet places the idle mixture adjustment screws at the base of the throttle plate in the front.

So, from this basic fact it should be obvious that properly setting the idle circuit to run as lean and efficiently as possible has multiple benefits. Right away, we can hear the detractors already warming up their keyboards stating that lean engines run hot. That, unfortunately, is a common engine-related urban myth. The exact opposite is true. Engines that run rich at idle tend to run warmer and those with retarded initial timing and rich mixture are the absolute worst at running hot. Those are facts.

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003 Edelbrock AVS carburetors place the idle mixture screws right in front near the base where they are easily accessed
Edelbrock AVS carburetors place the idle mixture screws right in front near the base where they are easily accessed.

Based on this, the first thing we need to address before setting the idle is to look at the ignition timing. Our experience is that many street engines do not take advantage of sufficient initial timing. So that’s where we will start.

Before we get into the details, it’s also important to state that this procedure is based on the assumption that the engine is in good condition with relatively even cylinder pressure on all cylinders with new or near-new spark plugs and plug wires. It’s also important that the engine is not suffering from even a minor vacuum leak.

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004 idle speed adjustment on Holley four barrel carbs is found next to the primary throttle linkage
The idle speed adjustment on Holley four-barrel carbs is found next to the primary throttle linkage. When performing adjustments, if the carburetor has a choke, make sure the choke is fully retracted and not sitting on the fast idle cam.

We should also state again that this story is aimed at the wide range of street engines with anywhere from stock to mild performance camshafts that idle with a manifold vacuum of 12 to 18 inches of mercury (Hg) as measured on a vacuum gauge. If your engine is running a big cam with less than 10 Hg, this will require different settings and carburetor adjustments (predominantly concerning the transfer slot), which requires a separate story to address the process.

Initial timing is the amount of ignition timing set at idle with the vacuum advance disconnected. Generally, older stock engines from the factory were often set at barely 4 to 6 degrees of initial timing. From a performance standpoint for even mild performance engines, they really prefer to run with 10 to perhaps as much as 15 degrees initial timing. This, however, sets up a situation where this much-added initial timing may produce too much total timing.

005 This simple illustration helps with visualizing how the idle circuit works
This simple illustration helps with visualizing how the idle circuit works. Fuel (with air mixed in from the idle air bleed) enters the channel from the top where it is metered by an idle feed restrictor (C). The upper opening in the venturi is the idle transfer slot (A) that supplies fuel to the engine as the throttle is opened slightly beyond the idle stop. The idle mixture screw only meters fuel to the idle feed port (B) that is located below the throttle blade.

This total is the initial timing value added to the mechanical advance. If we have 12 degrees of initial and 24 degrees of mechanical, this will produce 36 degrees of total timing. For most street engines, 36 to 38 degrees should be sufficient. Anything more than 38 to 40 degrees may create detonation problems. We prefer to set the initial based on attaining the highest idle vacuum then pull the timing back about two degrees. This seems to work well.

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For this story we will apply these techniques to a 355ci small-block Chevy with a mild cam, iron exhaust manifolds, and a dual-plane intake with a 750-cfm Holley four-barrel carburetor. We set the timing at 14 degrees initial. This produced 37 degrees of total advance. The procedure for setting idle mixture will be the same regardless of the carburetor in use.

006 The arrow points to the idle discharge hole located below the throttle blades
This photo is taken from below the throttle plate on a Holley. The arrow points to the idle discharge hole located below the throttle blades. With this used carburetor you can see where the fuel has cleaned the throttle bore as it discharges. The partially open throttle reveals the idle transfer slot that meters additional fuel until sufficient air speed can initiate fuel flow from the main metering circuit venturi boosters.

The first step is with the engine not running. Check the position of the idle mixture screws by counting the number of turns out from the seated position for each idle mixture screw. If the mixture screws are not the same count, average them and make sure they are an equal distance out from fully seated. They should be roughly 1 to 1½ turns out from fully seated but that’s just an initial setting. This is true whether the carburetor has two or four idle mixture screws.

Next, we need to connect a vacuum gauge and if your timing light has a digital tach readout that would be helpful as well. Start the engine and bring it up to its normal temperature and the choke is in the full “off” position. Idle speed should be somewhere between 750 to 900 rpm with the transmission in Neutral or Park.

007 Idle vacuum is expressed in inches of mercury (Hg)
Idle vacuum is expressed in inches of mercury (Hg). This particular vacuum gauge is a bit busy because it also lists pressure. The engine vacuum scale is on the lower half on the inboard side expressed as Hg and reads 12 Hg. The outboard scale is also vacuum but expressed in metric terms as centimeters of mercury.

Begin the adjustment process by noting the idle vacuum level and engine idle speed. Then move the first idle mixture screw in–clockwise–a very small amount. We like to work with adjustments that are about the width of the screwdriver slot. This is a very small change. Note any change on the vacuum gauge and/or tach. If after adjusting the first screw there is no change, make the same inward adjustment on the opposite idle mixture screw. This reduces the amount of fuel delivered by the idle circuit.

If there is still no change, make the same adjustment for both screws and note the change. If the idle speed drops and vacuum decreases, reverse the direction back to the original point and then turn both mixture screws outward this same small amount.

008 We use a Bosch digital timing light with a retard feature that can also display engine rpm
We use a Bosch digital timing light with a retard feature that can also display engine rpm. In this case, we’ve set the initial timing at 14.8 degrees before top dead center (BTDC). With the idle rpm at 650 rpm, the idle vacuum reading is very close to 16 Hg as the initial test. We’ll see if we can improve the vacuum reading by adjusting the idle mixture screws.

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Each time you make a change, note the effect on both the idle vacuum and engine rpm. If a change to the idle mixture increases the engine speed, go to the idle speed adjustment screw and bring the rpm back to its original point. This will slightly lower the vacuum level, too. The goal with this procedure is the highest engine idle vacuum at the desired idle speed. An example would be 14 Hg at 800 rpm.

009 We managed to improve the idle vacuum only slightly so this carb was already very close to ideal
Working slowly and methodically, make a small adjustment to each idle mixture screw and then evaluate the change with the goal of the highest idle vacuum at the same idle speed. We managed to improve the idle vacuum only slightly, so this carb was already very close to ideal.

Work with very slight changes to the idle mixture screws. This may take a few minutes to zero in on the ideal idle mixture setting, but it is well worth the effort. Once this is achieved, many tuners recommend making a very slight adjustment leaner from this ideal setting. This helps reduce the amount of fuel at idle, and in our tuning efforts we’ve seen how much this reduces the level of unburned hydrocarbons in the exhaust. This will also allow your spark plugs to run a little bit cleaner and produce less carbon in the combustion chambers.

It’s also important that each change is made the same way so that all the idle mixture screws are adjusted the same. In some cases, an engine might want more or less fuel on one side of the engine but this is rare. If you find that changes to one side of the carburetor have no effect at all, that circuit is likely blocked with dirt or debris and the carburetor should be completely disassembled and repaired. Sometimes you can try shooting carburetor cleaner down the idle air bleed holes in the top of the carburetor to clean the system.

010 Our Bosch timing light also offers a digital readout for idle speed
Our Bosch timing light also offers a digital readout for idle speed. By slowing the speed down and carefully adjusting the idle mixture to the highest rpm setting, this is a slightly more accurate process than using a vacuum gauge, but either way works well.

If, after running through this procedure, the idle mixture screws are turned out less than one full turn from fully seated, this indicates that the idle feed restrictor is slightly larger than ideal. This idle feed restrictor is the idle jet that determines how much fuel is fed to the idle mixture screws. By reducing this idle feed restrictor, this will also reduce the amount of fuel used at light throttle. With most carburetors, this is a difficult restrictor to modify. However, the results when you do lean this restrictor can be significant, especially to improving daily driving throttle response.

One indicator that the idle circuit is still too rich is if your engine will start easily and idle smoothly when it is dead cold without the use of a choke system. If so, the idle mixture is set too rich. When properly leaned out, a cold engine (with no choke) will require a couple of minutes (or more) of warm-up before it will idle without manually holding the throttle open.

011 To improve overall performance it’s a good idea to check the total advance at over 3 000 rpm without the vacuum advance connected
To improve overall performance, it’s a good idea to check the total advance at over 3,000 rpm without the vacuum advance connected. Then check with the vacuum advance. Total advance at 3,000 rpm with the vacuum advance connected and working could be well into the 45-degree range, which is a good starting point for high-vacuum part-throttle operation. In our case, we actually saw more than indicated here at 49 degrees BTDC at nearly 20 Hg manifold vacuum at 3,500 rpm. The engine runs fine with no detonation problems.

Of course, if any changes to the engine are made, like a different camshaft, the idle mixture will need to be readjusted and optimized. Another example would be changing from a pair of breathers to a PCV system will also require readjusting the idle mixture setting. As an aside, a good PCV valve should be considered an essential part of any street-driven engine.

012 Power Valve on Holley–Urban Myth
One urban myth worth dismissing is the idea that a properly functioning power valve (such as 10 inches of vacuum) can open at idle and cause the engine to run rich. The power valve is a separate circuit not connected to the idle system. If the power valve opens at idle, it will not deliver additional fuel until the main metering circuit is engaged under load. More importantly, generally a 6.5 Hg power valve is a much smarter choice for most mild street engines.

This procedure is not difficult and other than a vacuum gauge and a timing light, does not require any more specialized tools. And these are tuning tools you should already have. The main ingredient for setting proper idle is a willingness and patience to do the job correctly. The payoff is a smoother-running engine that will be happier and more fun to drive.

Tools
Small Straight Blade Screwdriver
Vacuum Gauge
Timing Light
Digital Tachometer

 

Tool Description PN Source
Bosch Dial-Back Timing Light 7521 Amazon
Vacuum Gauge INO-3620 Summit Racing

 

Source

Summit Racing
(800) 230-3030
summitracing.com

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