Induction / Q&A

Ask Away! with Jeff Smith: Should I Drill Air Bypass Holes In All 4 Carb Throttle Blades to Improve Idle?

Recently you wrote an article that mentioned drilling holes in all four throttle blades on the Holley carb to change the blade position compared to the transfer slot. I have a 393ci small block Chevy (3.58 inch stroke in a 400 block) in my 1967 Nova bracket car (9.49 second ET/139 mph). It has a single 0-4779 750 cfm Holley on an Edelbrock Super Victor manifold. As you might expect this has a big cam with a pretty rough idle.

So far I’ve drilled two 1/8 inch holes in the primaries, but the mixture screws are still unresponsive, which is what led me to enlarge the existing holes. With the carb off the engine for the winter I’ve seen the transfer slots are still way too exposed.

Would I want to drill all four throttle blades, or just the primaries?

Thanks in advance,

B.N.

The story that this question refers to outlined the practice of drilling small idle air bypass holes in the throttle blades of a Holley carburetor. The reason for doing this is related to an important yet often overlooked circuit called the idle transfer slot.

Most carburetors, including the Holley, employ a slot cut into the side of the throttle bore that begins just above the factory stock idle stop position. This idle stop position is set by the curb idle speed adjustment screw on the throttle linkage.

For most engines with a mild camshaft this curb idle position is fairly close to what is required. But engines fitted with long-duration camshafts lower the normal idle vacuum of 14-18 inches of mercury (“Hg) as read on a vacuum gauge. This lower vacuum is often less than 10 “Hg, and this requires a larger throttle opening to allow air into the engine due to the reduced signal from the engine.

When the curb idle setting is increased, this also uncovers more of the idle transfer slot. The transfer slot is designed to add more fuel to the engine in that short span of throttle opening between idle and when fuel is delivered by the main metering circuit through the boosters.

This transfer slot fuel is metered by an idle feed restrictor located upstream of the idle mixture screws. When the throttle blades are opened farther, this uncovers more of the idle transfer slot. This automatically adds more fuel to the engine, creating an overly-rich idle mixture condition. The shortcut solution is to lean out the idle mixture screws to compensate for this rich mixture. As our friend discovered on his engine, leaning out the idle mixture screws did not solve the over-rich condition.

Unfortunately, adjusting the idle mixture screws leaner (inward) often creates a lean hesitation under light acceleration that cannot be resolved with accelerator pump squirter changes or ignition timing. This hesitation is caused by the entire idle circuit becoming too lean just before the main circuit takes over.

The proper solution requires moving the primary throttle blades back to a position where very little or none of the transfer slot is exposed at curb idle. The old school way to solve this problem is to drill a small air bypass hole in each of the four throttle blades. This requires the tuner to drill four small holes and then place the carburetor back on the engine and evaluate the idle speed. If the speed is still too slow, slightly larger holes (in 1/32 inch increments) will need to be drilled. But you want to be careful not to drill the holes too large or the idle speed will be too high with the throttle blades completely close. A set of four 3/32 inch holes is a good place to start.

In this reader’s case, he’s working with a 750 cfm Holley double pumper carburetor with two idle mixture screws in the primary side only. But even in this situation, Holley supplies idle fuel through the secondary side of the carburetor even though it is not externally adjustable. They do this to meter fuel through the secondary side to keep the fuel fresh in the float bowl.

Since he’s already drilled two 1/8 inch holes in the primary side, the engine could use two small holes drilled in the secondary throttle blades side to bypass more air, close the primary side throttle position, and balance airflow through the intake. We would suggest starting with slightly smaller than 1/8 inch holes and see how much this will close the primary throttle position. If the holes are drilled too large, this will push the idle speed up perhaps past the desirable level even with the throttle blades completely closed.

Another trick that can help reposition the primary blades is to slightly open the idle stop position of the secondary throttle blades. On most Holleys, this stop is located in a very small adjuster on the bottom side of the base plate on, logically, the secondary side. Most mechanical secondary Holleys place this on the linkage on the opposite side of the primary linkage. Only make very slight changes to this opening.

Holley now offers the Ultra XP line of carburetors that have a built-in air bypass circuit that can be adjusted to set idle speed while leaving the primary linkage in the proper position. This air is pulled in through holes surrounding the air cleaner stud. The amount of air bypassed is adjusted through the air cleaner stud hole with a small straight blade screwdriver.

When the primary throttle blades are opened too far, they expose too much of the idle transfer slot (arrow). This allows additional idle fuel into the engine and will also create an off-idle hesitation. The large hole to the right of the slot is the idle mixture outlet port. (Image/Jeff Smith)
By drilling holes in the primary and secondary throttle blades (arrows), this will allow additional air into the engine and allow the tuner to close the blades and cover up the idle transfer slots to their proper orientation. Ideally, only a very tiny portion of the transfer slot is revealed—as shown here. (Image/Jeff Smith)
Most mechanical secondary Holley carbs come with an externally adjustable idle stop linkage on the passenger side of the carburetor. With normal Holley carbs, the carburetor must be removed and inverted to access the secondary throttle stop. (Image/Jeff Smith)
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8 Comments

  1. Can you please make your articles printable? Or tell me how to print them off.thanks

  2. What if the transfer slots are still over exposed and cam isnt too to big (like 224ish duration)? Timing at about 14 degrees at idle, no vacuum leaks that i can find…..

    • Jeff+Smith says:

      It’s possible that even with the small cam that something is requiring a larger throttle opening. It would be best to check engine vacuum before modifying the carb. Also, how much of the transfer slot is showing? If it’s less than 0.050-inch (that’s less than 1/16-inch) it’s okay and I would not worry about it. Are you experiencing an off-idle hesitation? If not, the transfer slot is probably ok.

    • Have you tried more initial timing? If you advance the timing and the idle RPM goes up, that’s your engine’s way of telling you it likes/wants more initial timing…and when you advance the initial timing you’ll end up closing the throttle blades to bring the RPM back down…each time you do that you’re closing off a vacuum leak so your vacuum will go up and your throttle mixture screws will become more responsive.

  3. Hello. This small holes can have the same function on a Ford 5.8 with TB ???

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