You Can Be a Carb-Tuning Hero

You'll need some electronics to do it, but now it's easier than ever to tune your carb like a pro.

By Jeff Smith

Photography by Jeff Smith

In a time not long past there was that guy down the street fromyou--that special car-crafting guru who could tune an engine by ear. Hewas revered by the local gearheads for his uncanny knack for creatingnot just impressive power, but also for transforming chugging streetslugs into razor-sharp sweetheart engines. He was like the PinballWizard--the deaf, dumb, and blind kid who could play those flipperfingers like he was part of the machine. We were all in awe of the CarbWizard.

Float level can be used as a tuning tweaker for subtle changes to air/fuel ratio. Lowe is leaner and higher is richer.

While that guy still exists, we've discovered a little piece ofelectronic technology that can turn almost any knowledgeable car crafterin a Carb Wizard. This latest development in handheld electronic-powerknowledge is the affordable air/fuel-ratio meter. While there are manyon the market, we first told you about a favorite, the InnovateMotorsports digital unit, in "Tune In, Turn On, and Make Power" (Feb.'04) where we explain how it works alongside a few basics concerningair/fuel ratio and horsepower. This is such a potentially great toolthat we thought we'd get into how to tune your engine using this toolnot just for max power, but also to raise the bar for betterpart-throttle response and highway cruising. While the Innovate meter iscapable of logging up to 44 minutes of brain-numbing data, we'llapproach this tuning session assuming you're going to read the Innovatemeter in real time. Ideally, data logging is better because you canstudy the information more closely.

WOT

The best place for wide-open throttle (WOT) air/fuel testing is at thedragstrip. Short of that, you can do Second-gear bursts of 2 to 3seconds each and have a passenger watch the meter. For your initialwork, you should shoot for WOT air/fuel ratios between 12.5:1 to 13.0:1.Remember to make only one change at a time and keep using the same testprocedure. If you're at the dragstrip, use mph numbers to help withtuning trends. As long as your changes improve trap speed, continue tomove in that direction. In many cases, the combination will berich--like 11.8:1 at WOT. That means you should start by leaning out thesecondary side of the carburetor. With Holleys and Demons, minimum jetchanges of two sizes per step are a good idea. For example, if you wantto run leaner, go from 80 rear jets to 78s. It's also a good idea tostart your tuning with the carb in its stock jetting configuration. Ifyou are five or six jet sizes (or more) away from the box-stockconfiguration, it's possible there's something else wrong with the carbor your engine.

Part-Throttle Tuning

Contrary to what you may think, street engines spend a majority of timeat idle and at very low throttle openings. In addition, the idle circuitcontinues to deliver fuel even when the carb is delivering fuel throughthe main metering circuit. Given this, the best place to start improvinghighway and in-town fuel mileage is with the idle circuit.

A high-speed lean condition can often be traced to low fuel pressureresulting from an underperforming fuel pump.

Most general-purpose aftermarket performance carburetors are designed todeliver around a 12.5:1 air/fuel ratio to avoid lean surge conditions.Most mild street engines can tolerate part-throttle air/fuel ratios of13.5:1 all the way up to as high as 15.0:1. Keep in mind that allproduction EFI engines operate at 14.7:1 air/fuel ratio and thedriveability is excellent. It's more of a challenge to tune a carburetorto achieve a lean 14.7:1 air/fuel at part-throttle and still deliverexcellent and immediate WOT power, but it can be done. What this meansis the power-valve and accelerator-pump circuits become much morecritical. This is where a Q-jet shines, using its small, highlyresponsive primary side to achieve excellent throttle response forpart-throttle driving. But other carbs can be tuned to also work verywell. Don't be afraid of 14.0:1 or even 14.5-15.0:1 air/fuel ratios forpart-throttle highway cruising. While these are lean ratios, they don'thurt the engine. There is very little load at highway cruise speedsbecause the engine is only making about 15 to 25 hp under theseconditions.