Unlike an EFI system, a carburetor is a purely mechanical device that meters air and fuel in the proportion needed for combustion. While this might seem simple enough, a carb takes advantage of a handy characteristic of nature identified way back in 1738 by the physicist Daniel Bernoulli called the Bernoulli Principle, better known as the Venturi Effect. The key aspect of the Bernoulli Principle is the inverse relationship between velocity and pressure, which dictates that as the air speed increases through a carburetor’s venturi, the pressure will drop. This lower pressure is just what is needed to pull fuel out of the carb’s bowl, which is at atmospheric pressure. The beauty of a carb is that its operation is based on the simple properties of physics. Harnessing the velocity-dependent pressure drop to meter air and fuel in exactly the needed proportions over a wide range of operating requirements relies on a complex system of calibrated orifices, and those are the items that comprise what we refer to as the carb’s calibration, or "tune." What is actually being calibrated here is how the carburetor as a metering device will respond to changing pressure differentials in its various circuits.
The level of carburetor tuning that an enthusiast can practically perform will depend upon the design of the carb, and the tuning components available. The most basic level here is a backyard mechanic armed with nothing more than a screwdriver and a good ear to attack his OEM carb. With a little talent, this guy can adjust the idle mixture and speed, and call it a day. The Holley modular carbs were a game changer in the world of carb tuning by adding just one additional tuning point for the average enthusiast: the main jet. The main jet is, as the name implies, the main metering orifice that restricts fuel flow into the main metering circuit. The Holley carb provided ready access to the main jets and the company supported the ability to tune the main jets with readily available replacement jets and kits. Sure, most other carbs featured replaceable main jets, but with limited replacement jet availability and accessibility, few bothered. The Holley carb brought jet changes trackside.
Of course there are many other tuning points on a traditional Holley carb, but with the earlier units these were regulated by fixed orifices. Here, carburetion gurus practiced their black art by drilling and sometimes filling these occult tuning points, whereas the average guy went no further than the familiar jet change. While that may have been enough, higher-end modern carburetors have taken carb tunability to an extreme level. These days we find replaceable metering orifices at virtually every tuning point of the carburetor, giving the potential to calibrate virtually every aspect of the carb’s metering. That alone might be too much of a good thing, but with the addition of readily available lambda meters capable of detailing air/fuel ratio in real time, we have both the ability to make advanced tuning changes, and the instrumentation to readily quantify those changes.
Just because you can change so many of the calibration points of a Holley carb does not necessarily mean you should. To start, Holley puts extensive work in the base calibration of their carburetors to fit their intended applications. Experience here over many years of testing and data logging has shown that they do a very good job. Chances are, unless you are serious about a methodical approach to dialing in the carb, or are looking to address a specific aspect of its metering, sticking to simple jet changes will handle the majority of carb tuning requirements. That said, pick up a new HP-series carb, and those tuning points are there for the tweaking. If you are contemplating getting into this level of advanced carb tuning, you had better know exactly what those tuning changes do and how they affect the carb’s metering. To illustrate the causes and effects, we’ll go through the key tunable metering points, their function, and how tuning changes affect fuel metering.
1. Idle Speed Screws
You might as well forget about getting into the exotic aspects of carb tuning if the simple stuff is all fouled up. One is the most basic of carb adjustments is the idle speed setting. This is simply setting the position of the throttle plates via a mechanical stop at the adjustment screw, thereby setting the idle speed. You’ll find an idle speed adjustment on almost every carb, but adjusting it properly involves a little more than just reading the tach. A properly calibrated carb should idle just off the idle circuit, therefore you should not see fuel dribbling from the carb booster at idle. The engine idle speed is a function of several variables, from the ignition timing to the air/fuel ratio, to the amount of air getting in.
Though all Holley carbs feature nearly identical circuits, how the carbs are equipped will affect the tuning ability. A base street carburetor such as this Holley Street Avenger is factory equipped with fixed drillings for the air bleeds, power valve channel restriction, idle feed restriction, and emulsion bleeds. Basic tuning here is limited to accelerator pump components, jets, idle circuit, and float level.
A carb's idle mixture screw at three-quarter turn out from lightly bottomed at all four corners serves as a good baseline bench setting. The running engine should be responsive to changes in the idle mixture setting from here.
A look down the throat of a Holley Ultra HP shows the replaceable air bleeds; note the drillings in the center tower routing air to the air bypass adjustment under the air cleaner stud.