Transition slot exposure is critical because it affects both idle quality, idle air / fuel mixture as well as 'tip-in' or 'off-idle' response. Too little or no transition slot exposure on the primary side on most engines will cause a stumble or hesitation on tip-in. Viewed with the butterflies in the idle position, base inverted. Most carburetors will have the secondary butterflies adjusted with a small length (approximately 0.020-inch, appearing as a square) of the idle transfer slot visible below the bottom edge. If the secondary transfer slots are uncovered excessively as shown here, the engine can develop an undesirable hesitation when the butterflies are suddenly.

1. Holley Idle Transfer Slot Adjustment
2. Holley Idle Transfer Slot Restrictor
3. Holley Idle Transfer Slot
4. Holley Idle Transfer Slot Modifications

In this episode of Unity Motorsports Garage we continue with our Holley Carb Tuning series by driving the 509 BBC powered ElCamino to see how our Idle mixtur. That will set up your transfer slots for best efficiency at idle. You can further tune it by doing the wire in the IFR trick, or if that makes your teeth itch, change out the IFR for smaller ones. FYI, Holley themselves suggest using the wire, so it is not so much a bandaid fix as it might seem.

At the risk of sounding repetitive, let me say once again, that if you bought an application-specific carb, you probably are only going to need to do minimal adjustments to mixture calibration. You are only likely to run into a problem with the idle and transition calibrations if the carb you have is way off spec for the application.

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As an example, I had a 10.5:1 350-ci race engine that was a stout runner but the class it was running in changed engine rules. I felt this engine would make a great street engine. The 830-cfm carb (modified 750) was a heavily reworked piece and was top-notch for the race application. This engine’s transformation to a street engine took little more than a change of cam from a big race grind to a moderate street grind. The difference was an idle vacuum of about 4 to 5 inches for the race cam and 11 to 12 inches for the street cam. Among other things, the successful conversion involved resizing the idle jet and replacing butterflies, which had excessively large idlebypass holes in them. In addition, smaller accelerator pump squirters and a pump cam were installed; a mechanical secondary linkage that delayed secondary opening as long as possible was used. The result was about 490 ft-lbs of torque and horsepower just shy of 515.

These are the idle mixture adjustment screws (yellow circles). Although they have the biggest influence on final setting of the idle mixture, they are by no means the only factor.

This chapter provides what you need to know to make good on the idle and transition circuit to allow almost any carb to work for your application or simply to fine tune one that is just short of perfect.

Idle Mixture Calibration

At this point, you should have a running engine (see section “Idle Mixture” on page 29 in Chapter 3). With the engine at operating temperature, you can begin. Assuming that an oxygen sensor is not part of the tune-up equipment everything needs to be done visually by reading the tailpipe smoke or a vacuum gauge, or simply by listening. Your first move is to attach a good vacuum gauge to a manifold vacuum source (that is, one originating from beneath the throttle butterflies).

Check the fuel levels in the fuel bowls and adjust as necessary (see Chapter 12 for details). If the original settings are still in place, the idle mixture screws should be two turns out from the seated position. If the idle jetting is about right this setting should be delivering a mixture that tends toward the rich side (see Figure 2.21 on page 21). If you are serious about tuning the idle and transition circuits for best results, especially maximum fuel efficiency usage, you start to appreciate a carb with replaceable air bleeds in the carb’s main body. Here are the steps for calibrating the idle mixture: Progressively turn the idle-mixture screws in. Start with a quarter turn on each one and readjust the idle speed to an appropriate RPM. A big-cam engine needs a higher idle speed, which is usually about 1,000 rpm; a regular street cam needs about 600 rpm.

Virtually all cast base-plate carbs have a vacuum port (arrows) connected to the underside area of the butterfly to read manifold vacuum. Billet base plates most often do not, so you need to drill the intake manifold and install a fitting. Note the size of the vacuum gauge used here. This is about the minimum size you should use.

Holley Idle Transfer Slot Adjustment

Idle feed restriction (idle jets) can be located in different positions for different metering blocks. On the left (red arrow) is the fixed type most commonly seen in regular metering blocks. On the right (yellow arrow) is the replaceable style for the Ultra range of Holley carbs.

Replaceable idle air bleeds (yellow arrows) are used in the top-of-the-line street and race Holley carbs, making a change easy. If you have a regular carb, the air bleeds are press-in (red arrows) items, so changing them is limited to drilling larger or pulling them out and installing smaller ones.

Here the base plate is backlit to show how much slot the butterfly should uncover at idle. The maximum is 0.04 to 0.06 inch, with closer to the lower limit being preferable.

Holley’s Ultra models have an adjustable idle air bypass screw (yellow arrow). Air for the bypass goes through the holes indicated by the blue arrows.

To supply the required amount of idle air without using up all the transition slot, it may be necessary to drill holes into the butterflies.

The primary idle speed adjustment screw is common to virtually all carb models.

The secondary idle speed adjustment (arrow) is done from the underside of the regular cast base-plate models.

Here, some idle-speed experimentation is worthwhile. Try adjusting the RPM to the point the engine is just short of stalling. When you have established this RPM, set the idle speed to about 100 rpm higher. Continue progressively adjusting the mixture screws in until you achieve the highest manifold vacuum. As the mixture nears optimal, you may need to readjust the idle speed again, as an optimal mixture also allows a lower, stable idle RPM. If the idle mixture screws make little difference to the idle from the full-in to the full-out position, it’s a fair bet that the butterflies are too far into the transition slot. If the idle jetting is okay, the idle mixture screws should ideally be about one turn out, although 1/2 to 11 ⁄2 turns is acceptable.

Next, check the position of the idle speed adjustment screws. As per Chapter 4 you started with these at two turns into, opening the butterflies of the primary (or primary and secondary). If a satisfactory idle speed is achieved with the idle speed screws less than two turns in you are looking good. If it takes more than two turns you may be on the verge of, or even into, using up too much of the transition slot. If this is the case the engine probably exhibits a stumble just prior to coming on to the booster-driven main jet circuit.

You can perform a cursory check to determine the existence of two potential issues: idle jetting size and the transition slot usage.

Idle Jetting Size

One way to check on jet size is to use the 3,000-rpm test. You slowly open the throttle so the accelerator pump is not brought into action. This establishes whether the engine runs cleanly to 3,000 rpm without a hesitation or misfire. If it does, the setup is at least close to the requirement. If the engine stumbles and the mixture screws are more than 11 ⁄2 turns out, it is a sign that the mixture is going lean. If this is the case, the idle jet needs to be larger or the air corrector smaller. A good test is to stick a wooden toothpick into one of the idle air corrector jets to see if it helps with the 3,000-rpm test. Although rarely the case, be aware that this may richen the circuit too much and the engine now stumbles because it is too rich.

Because the air corrector jet is more accessible and quick to change, I usually rejet here rather than at the idle jet. With either jet, resizing should be done at about two thousandths at a time. If the carb is not equipped with replaceable jets, insert a fine piece of fuse wire (with a bend in it so it does not go all the way into the idle well) to block off some of the air going into the air corrector jet. If you have a fixed-idle jet, resizing is best done with a pin chuck and a jet drill set.

Transition Slot Usage

Before you attempt any idle jet resizing, be sure to look into the possible alternate issue that can lead to an off-idle stumble. It could be that too much of the transition slot is used up to get an acceptable idle; although this problem tends to show up more often when putting the engine under a load commensurate with a low-speed cruise. If too much of the slot is uncovered at the idle position there is insufficient slot length to effectively carry through from the transition to the main circuits. About 0.060 inch from the carb’s underside should be regarded as an absolute limit.

Excess transition slot usage also causes the idle mixture screw adjustments to be insensitive. This is only possible if the cam is bigger than a typical stock one. So, because of the reduced vacuum, you need to increase the flow area available through the carb while at idle. Drilling a small hole in the primary butterflies takes care of it.

Start with a 1/16-inch hole and work your way to about 1/8 inch. If the problem has improved but not completely cured, start drilling the secondary butterflies. Be aware that only a big-cammed all-out race engine requires as many as four 1/8- inch holes. If the carb has an adjustable idle air bypass located under the air filter stud, this hole drilling exercise is redundant. If too much transition slot is uncovered, open the idle air bypass more so the throttle butterflies can be more nearly closed.

Oxygen Sensors

If you have oxygen sensor mixture measurement, the idle calibration is a whole lot easier. To do the idle/transition calibration, you go through the same process as described above, but you have the benefit of knowing what the air/fuel ratio is at any given moment. The question most often asked here is, What ratio should be used for idle? Which ratio gives the best results tends to vary from one engine to another. You should tune for the leanest air/fuel ratio that provides the desired idle results. For the most part, you find that high-compression short-cammed engines with efficient exhaust systems run the leanest while still producing good idle characteristics. Engines with big cams tend to want more fuel so you should run a richer mixture for a good idle. Most engines fall into the spectrum of 13.0 to 14.0:1 although an engine targeting economy may well, in my experience, be able to run as lean as 15:1.

On billet base-plate carbs, the secondary has an adjustment screw on the top as well as underneath.

Road Test

Now is time to put your tuning skills to the test. You may not want to go to the extremes detailed in Chapter 5 in an effort to get maximum fuel economy. But you should verify that your calibrations are doing a respectable job. Take the vehicle onto a flat road and test the calibrations from idle through transition to the main jet system and make sure the carb is functioning as required. To do so, very slowly depress the throttle so as to avoid any pump jet action. The engine should drive smoothly throughout the speed range from zero to 60 or 70 mph without hesitation. Note the oxygen sensor readings as the throttle opens and speed builds. The mixture should not be any richer than 14:1 but if everything is good in terms of the engine spec and condition, you should see air/fuel ratios in the 15 to 16:1 range. The tests should be conducted in high gear and up to about 45 mph. Anything over 17:1 produces a lean miss; that is, unless the engine is specifically built with the intent to fire super-lean ratios. If the engine develops a lean miss, your first move is to reduce the size of the idle air corrector by two or three numbers. If that does not fix the drivability issue, increase the idle jets by a number or two until it is resolved.

If the engine is for a race-only machine, maximizing fuel economy is not an issue. All you need to do is make sure that the idle quality is acceptable and that the low-speed drivability is all it can be. A point worth mentioning is that if the idle mixture and speed are optimal, the engine has less of a tendency to stall if the clutch is released at too low a speed. A good idle setup makes it much easier to move around the paddock at a race.

Written by David Vizard and Posted with Permission of CarTechBooks

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Holley Idle Transfer Slot Restrictor

Holley Idle Transfer Slot

Holley Idle Transfer Slot Modifications