t’s the beginning of a fantastic day. You’ve tightened the last bolt to install that bad-boy small-block in your Chevelle and it’s time for a testdrive. The engine is just plain gnarly at idle, sounding more like an NHRA Comp Eliminator small-block in the staging lanes rather than just another street small-block.

But on the testdrive, the engine exhibits a nasty off-idle stumble, and you notice that the idle is stinky fat but fiddling with the idle mixture screws does not seem to help. And when you lean out the idle mixture screws, that off-idle stumble gets worse despite adding larger Holley accelerator pump nozzles and a more-aggressive pump cam. Even your brother-in-law, the self-proclaimed Holley expert, has no clue on how to remedy this situation.

But there is a simple solution that anyone can apply to virtually any engine suffering this fate. It will require some tuning patience, but the actual steps are fairly easy. We’ll also offer some alternatives if you’re not sure about your prowess with making modifications to a typical Holley carburetor. This is the carb that we will focus on, although as mentioned, the steps would be similar for any carburetor.

To set the stage, we’ll use a hypothetical small-block Chevy engine that is running a fairly long duration camshaft with plenty of overlap–the kind of cam that produces that enviable choppy idle. Along with that unsteady idle comes a low intake manifold vacuum level. Our suggested crossover point where these tuning changes will be necessary is manifold vacuum of less than 10 inches.

Holley carbs are mainly set up to handle engines that idle above 10 inches of mercury (Hg) of idle vacuum. When a lower idle vacuum occurs, most Holley carburetors will require turning the idle speed screw clockwise to set a proper idle speed. By opening up the curb idle speed position on the throttle plates this uncovers something called the transition slot.

The transition slot is a thin machined opening in the primary throttle bore on most carburetors. As the throttle is opened during normal driving, this slot delivers additional fuel from the idle circuit that ensures the engine does not run lean before there is enough airspeed through the venturis of the primary side of the carburetor to pull fuel from the main boosters. With an engine idling at 12 to 15 Hg of idle vacuum, the throttle plates are nearly closed, and the transition slot is either barely exposed or completely closed.

Engines with low idle vacuum require the throttle plates to be opened more to allow sufficient air into the manifold for the engine to idle at the desired rpm. This creates the problem. As more of the transition slot is exposed, this adds more fuel that is not metered by the idle mixture screws. Even if the idle mixture screws are nearly closed, the air/fuel ratio may still be rich. Worse yet, with much of the transfer fuel delivered now at curb idle, when the throttle blades are opened past idle during normal driving, the engine will experience a stumble since the circuit now transitions from too rich to excessively lean.

So how do we fix this?

The easiest way to prevent this from happening is to not build an engine that idles with 6 Hg of manifold vacuum. But that’s not a realistic solution. The next least-effort opportunity is to buy one of the high-end Holley Ultra-XP line of carburetors that range from 600-, 650-, 750-, and 850-cfm versions. These carburetors feature an idle air bypass feature that allows the tuner to adjust idle speed without moving the position of throttle blades through a separate air bypass system. This maintains the proper throttle blade position relative to the transition slot and eliminates the off-idle stumble.

The only problem with this solution is that these are $1,000-plus carburetors, which are probably out of the reach of a typical street engine builder or tuner. But there is another solution that will only cost time and maybe a carb gasket or two.

The most popular and time-honored solution is to drill holes in the primary (and often secondary) throttle blades. This is a dedicated solution in which once the holes are drilled there’s no turning back from this approach. So, we should first run through some helpful recommendations to try before resorting to this serious modification to an existing carburetor.

Before we get into modifying the carburetor we should acknowledge the old tuner’s adage that states, “90 percent of all carburetor problems are ignition related.” That may over-state the case a bit but it’s worth recognizing the point. The first suggestion is to ensure the ignition system is working properly. This includes the distributor mechanical and vacuum advance systems. It’s also critical that the initial timing is set to a minimum of 12-15 degrees.

Engines with big camshafts combined with less than 10:1 compression suffer from low cylinder pressures at idle. To compensate, these engines demand much more initial timing compared to engines with 9.5:1 or more compression with less cam overlap. There are no hard and fast rules here except that engines with more overlap need more initial timing.

Also check to make sure the engine is not suffering from fouled spark plugs, a blown power valve, a vacuum leak or two, or that it has other mechanical problems like poorly adjusted valve preload or lash that would affect the engine’s idle vacuum. Assuming these systems are all in good working order, we can proceed with the carb modifications. First, carefully record the engine’s current idle speed—like 900 rpm in neutral, the idle vacuum, as well as the position of the idle mixture screws.

The idle mixture screw position also demands some attention. With the transition slot exposed, the idle mixture screws are often turned either more than two turns out or they have been adjusted to where they are only a half-turn out from fully closed. Both of these adjustments are improper. It might be worth it to set the idle mixture screws at one full turn out from fully seated to start with unless the engine refuses to idle. But we’ll get to that once we have the transfer slot correct.

The first and most important clue is to remove the carburetor and turn it upside down and check the position of the primary throttle blades relative to the transition slot. If the transition slot is exposed much more than the width of the slot, then this is a major indicator of a problem, especially if more than 3⁄16 inch of the slot is exposed.

Assuming this is the case, the next step is to drill a 1⁄8-inch hole in each of the primary throttle blades centered on the slot side of the throttle blade. Carefully drill these two holes and remove any chips from the carburetor. One way to prevent the chips from entering the carb is to coat the drill bit with grease, which will retain these chips. Do not change the position of the throttle blades with the curb idle screw.

Place the carb back on the engine and you should notice the engine idles at a slightly higher rpm. It’s also possible that the engine may not respond. If so, increase the hole from 1⁄8 to 3⁄16 inch. This should produce a change in idle speed. Assuming this happens, close the throttle blades with the curb idle screw and then remove the carburetor and inspect the position of the primary throttle blades to the transition slot.

What we want to achieve is to close the throttle blades to where the transition slot is barely exposed by the throttle blades or fully blocked. If holes larger than 3⁄16 inch do not help, you may consider drilling two more 1⁄8-inch holes in the secondary throttle blades to increase the volume of air entering the manifold.

Be very careful here because it’s very easy to overshoot with too many large holes. When this happens, the engine will idle too fast with the curb idle adjustment completely closed. If this happens, the best solution is to replace the throttle blades and start over.

It’s also important to readjust the idle mixture screws after increasing the bypass hole sizes. These will be minor adjustments, assuming that we have now achieved a proper idle speed with the transition slot nearly closed, this now separates the transition slot from the idle mixture screw circuit, which is how Holley designed the circuit to work.

Now we must readjust the idle mixture screws to achieve a lean idle. Enthusiasts often mistakenly think that a lean idle mixture will contribute to increasing coolant temperatures at idle. This is entirely false. In fact, an overly rich engine will tend to run hotter because the rich air/fuel mixture continues to burn as the exhaust valve opens. A lean mixture at idle burns very quickly and is almost entirely combusted by the time the exhaust valve opens.

Let’s assume the idle mixture screws are turned out to around 1 1⁄2 turns or more. It’s not uncommon to find attempts to fix the off-idle stumble with a richer idle mixture setting. Start by ensuring that all idle mixture screws are set the same number of turns out from lightly seated. It is important to always make the same adjustment to all the idle mixture screws.

With the engine idling, make very small adjustments of 1⁄16 turn or less for each idle mixture screw. Make a small adjustment to each idle mixture screw and then evaluate the change on the vacuum gauge shooting for the highest vacuum reading. After this highest vacuum reading is achieved, this will also increase the idle speed slightly. This is why it’s important to readjust the idle mixture screws after drilling the bypass holes to not overshoot the desired idle speed with too much bypass air. The trick is to keep the bypass holes as small as possible.

If the carburetor had been messed with prior to these adjustments, consider going back to the original accelerator pump nozzle size and return to the stock accelerator pump cam. Also make sure there is no slop between the accelerator pump lever arm and the accelerator pump. Excess clearance here can create a slight hesitation off idle.

As you can see, this process is quite lengthy but not necessarily complex. The key is to make each step carefully while working toward the goal of minimizing the size of the exposed portion of the transfer slot. Once this is achieved, the engine will respond with a leaner idle mixture setting and the off-idle stumble should disappear. The end result is a much happier engine at both idle and initial throttle opening. This will likely also put a smile on your face.