Holley Carburetor Tuning - 10 Power Moves

2. Before You Go To The Chassis Dyno
Although we never race with the engine at idle, tuning for a good idle is a useful tool toward refining the transition circuits that are used during yellow flag laps. Here, I am assuming there is a four-corner idle setup on the carb. The absolute first move is to set the fuel level exactly where it is supposed to be.

Next, we need to minimize fuel foaming. One step toward doing this is making sure the fuel pressure is no higher than it needs to be. A large needle and seat and low fuel pressure work better than a small needle and seat and a higher fuel pressure. It's worth noting that the Carb Shop's bottom feeder needle and seat assembly is far less prone to power-robbing fuel foaming. We also need to make sure that the secondary butterfly is open sufficiently so that it passes its share of idle air.

With preliminaries taken care of, it's time to establish good transition calibration. The transition of a Holley carb is run from the idle jets, so this makes idle calibration more important than you may have thought. With the engine at running temperature, run the idle speed down as low as it will go. Then check the position of the idle screws. They should be about 31/44 to 1 turn out. If the screws' idle positions are less than the idle/transition, calibration is basically too rich. If it is out by more than, say, 111/42 turns, it is too lean. Going up on the idle air corrector size will make the circuit lean, and going down will richen it. The opposite is, of course, true for the idle fuel jet.

3. Ignition Timing
As much as it seems divorced from carb calibration, ignition timing can have a lot to do with how responsive an engine is as it transitions onto the main circuit. It also can affect fuel consumption on yellow flag laps by as much as 50 percent. At idle, a hot street engine with vacuum advance may well require as much as 50 degrees of ignition advance, with 45 being common. If this is what the engine wants, then it will idle at a lower rpm with less throttle opening than if it had only 10-14 degrees of ignition advance out of the available centrifugal advance working for it.

With the butterflies closed more, more of the transition slot is available. Also, under the part-throttle conditions seen during yellow flag laps, the manifold vacuum being produced may indicate that the most fuel efficient timing is about 45 degrees. This is about 15 degrees shy of what is typically seen with centrifugal advance alone. You need to look long and hard around the infield to find a race engine with vacuum advance on it, but you will find it on mine. If you want to be a pioneer here, then take a good look at Crane's electronic distributor or give Performance Distributors a call.

4. Block To Bowl
Leaks It is important to have a perfect seal between the float bowl, metering block, and carb body. As obvious as this may seem, a lot of minor calibration problems can arise due to fuel bleeding internally across circuits within the carb while it seems none of it leaks out of the float bowl.

There are two things to do here: The first is to establish that the components involved are not warped (believe me, this is not an unlikely scenario). The second thing is to always have on hand a good stash of new float bowl/metering block gaskets. When repeated float bowl removals are the order of the day, it's best to use oil or WD-40 on the gaskets so they don't stick. Just for the record, Don Gould at FBO Systems has some reusable (20 times at least) gaskets that can make life a lot easier here.

5. Power Valve And Stagger Jetting
Because the main jet system of a Holley carb has been misnamed from day one, it has falsely led most users into believing that the main jet is the main jet when, in fact, it is the cruise mixture jet. In reality, the part-throttle cruise mixture should be set by sizing the "main jet," and the wide-open throttle (WOT) mixture should be set by calibrating what is commonly called the power valve restriction channel (PVRC).

The first part of the deal here is to make sure your carb's metering blocks have PVRCs that can be calibrated. If you are not up to speed here, you can get all you need from Holley or an independent carb specialist such as Advanced Engine Design (AED).

When the main jet and PVRCs have been calibrated to give a good cruise and WOT mixture, it is OK to fine-tune the main jet. This is especially so if the carb is calibrated to give near-optimum fuel distribution around all the intake port runners. This inevitably means different sizing from front to back and side to side. So what's all this fine-tuning worth? Well, given an O2 sensor in each exhaust pipe, (forget those exhaust temps-they are close to useless for this) the best gain I have seen from finely tuned stagger jetting was no less than 16 hp on a 520hp motor, but about 5 hp is a more normal figure.