Become A Carb-Tuning Hero

You’ll Need Some Electronics to Do It, but Now It’s Easier Than Ever to Be A Carb-Tuning Hero

By Jeff Smith

In a time not long past there was that guy down the street from you; that special car-crafting guru who could tune an engine by ear. He was revered by the local gearheads for his uncanny knack for creating not just impressive power, but also transforming chugging street slugs into razor-sharp sweetheart engines. He was like the Pinball Wizard – the deaf, dumb, and blind kid who could play those flipper fingers like he was part of the machine. We were all in awe of the Carb Wizard. While that guy still exists, we’ve discovered a little piece of electronic technology that can turn almost any knowledgeable car crafter in a Carb Wizard. This latest development in hand held electronic-power knowledge is the affordable air/fuel-ratio meter. While there are many on the market, we first told you about a favorite, the Innovate Motorsports digital unit, in Tune In, Turn On, and Make Power (Feb. ’04) where we explain how it works alongside a few basics concerning air/fuel ratio and horsepower. This is such a potentially great tool that we thought we’d get into how to tune your engine using this tool not just for max power, but also to raise the bar for better part-throttle response and highway cruising. While the innovate meter is capable of logging up to 44 minutes of brain-numbing data, we’ll approach this tuning session assuming you’re going to read the Innovate meter in real time. Ideally, data logging is better because you can study the information more closely.


The best place for wide-open throttle (WOT) air/fuel testing is at the dragstrip. Short of that, you can do Second-gear bursts of 2 to 3 seconds each and have a passenger watch the meter. For your initial work, you should shoot for WOT air/fuel ratios between 2.5:1 to 13.0:1. Remember to make only one change at a time and keep using the same test procedure. If you’re at the dragstrip, use mph numbers to help with tuning trends. As long as your changes improve trap speed, continue to move in that direction. IN many cases, the combination will be rich¬† like 11.8:1 at WOT. That means you should start by leaning out the secondary side of the carburetor. With Holleys and Demons, minimum jet changes of two sizes per step are a good idea. For example, if you want to run leaner , go from 80 rear jets to 78s. It’s also a good idea to start your tuning with the carb in its stock jetting configuration. IF you are five or six jet sizes (or more) away from the box-stock configuration, it’s possible there’s something else wrong with the carb or your engine.


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

Most general-purpose aftermarket performance carburetors are designed to deliver around a 12.5:1 air/fuel ratio to avoid lean surge conditions. Most mild street engines can tolerate part-throttle air/fuel ratios of 13.5:1 all the way up to as high as 15.0:1. Keep in mind that all production EFI engines operate 14.7:1 air/fuel ratio and the driveability is excellent. It’s more of a challenge to tune a carburetor to achieve a lean 14.7:1 air/fuel at part-throttle and still deliver excellent and immediate WOT power, but it can be done. What this means is the power-valve and accelerator-pump circuits become much more critical. This is where a Q-jet shines, using its small, highly responsive primary side to achieve excellent throttle response for part-throttle driving. But other carbs can be tuned to also work very well. Don’t be afraid of 14.0:1 or even 14.5¬† 15.0:1 air/fuel ratios, they don’t hurt the engine. There is very little load at highway cruise speeds because the engine is only making about 15 to 25 hp under these conditions.

This is definitely cut-and-try type work. IF you go too lean on the idle-feed restrictors, the engine will surge at cruise and hesitate under light acceleration, since most engines don’t like to accelerate at lean air/fuel ratios. If you tune the idle-feed restrictors too lean, the engine will most likely suffer from an off-idle lean stumble as an early indicator that you’ve gone too far. It’s the transition circuits that are more seriously challenged and the ones that will falter first when you begin to lean out the idle and primary main circuits.


Our first real-world example started out with a 9.0:1 compression 455ci Olds with a mild hydraulic cam, dual-plane Edelbrock, the stock 455 Q-jet, headers, and an HEI ignition. The Innovate meter reported an idle mixture of around 12.0:1 that went lean at about 13.8:1 as the throttle opened up in mild acceleration. At steady-state highway cruise speeds, the air/fuel ratio went back rich at around 12.5:1. At WOT, the engine was actually a bit lean.

The first thing we did was to adjust WOT tuning by swapping the stock CH secondary rods to thinner rods (eventually a pair of aluminum rods) until we had a WOT air/fuel that came in around 12.8:1. Because of the cam, intake, and header swap, part-throttle transition fuel was also slightly lean, so we changed to a slightly weaker power- valve spring to pull the metering rods out of the primary jets sooner as load increased. The engine responded with a slightly richer mixture in light acceleration of around 13.2:1, but we still had a slightly rich cruise air/fuel ratio. When we tried larger (leaner 52B) primary metering rods, it hurt light-throttle acceleration. Some late-model Q-jets are set up to allow you to adjust the position or depth of the primary metering rods in the jets. If our carb had been equipped with this feature, we could have adjusted the metering rods deeper in the primary jets to lean out the part-throttle metering.


We also tried a 350 Chevy equipped with a long-duration cam and a Holley 750-cfm 0-3310 vacuum-secondary carb that suffered from a bad off-idle stumble and a pig-rich 10.5:1 ratio at part-throttle cruise. The WOT air/fuel ratio was only slightly rich at 12.2:1. The challenge was to improve the driveability and mileage without sacrificing WOT power. After properly adjusting the primary accelerator-pump linkage, the stumble disappeared. We also replaced the large 0.036-inch accelerator-pump nozzle to a 0.028 to further improve the throttle response.

The LM-1 meter told us the engine was way rich at just off idle, so we first tried leaning out the idle-mixture screws, but it didn’t help. Next we disassembled the carb and found that a previous hacker had drilled out the idle-feed restriction of around 0.032 to 0.035 inch. Some quick math revealed that the 0.052-inch orifice increased the area by more than 100 percent. As a temporary fix, we tried a 0.020-inch-diameter wire stuffed into the idle-fuel jet to reduce the flow area. The larger wire reduced the area by roughly 15 percent, which leaned out the part-throttle air/fuel ratio to roughly 11.8:1. which was still very rich. Further experimenting with reducing the main jets from 72 to 69 and using a slightly larger 0.025-inch wire in the idle-feed jet finally got the part-throttle air/fuel ratio close to 12.8:1. We could not tune any leaner without creating an off-idle stumble and part-throttle lean-surge condition because of the cam. The WOT air/fuel also improved the leaner primary jetting to 12.5:1. These simple changes improved the fuel mileage by over 30 percent.

An ideal accelerator-pump shot is just enough to maximize acceleration. Additional fuel only kills power.

The best approach is to weld the supplied bung into the exhaust pipe near or in the header collector. Mount the Bosch sensor in the upper half of the pipe to prevent condensation from affecting sensor accuracy.

Part of the Aux Box software allows you to custom configure up to five gauges on your laptop such as air/fuel, an rpm signal, and a total of four other 0-5-volt analog channels for stuff like voltage, engine temperature, engine vacuum (MAP), and throttle position. This data can be viewed in detail to help you dial in the car.

WOT tuning is relatively simple. If the meter reads richer than 12.5:1, remove the secondary float bowl on your Demon or Holley and try jets two steps leader. Shoot for between 12.5 and 12.9:1 air/fuel or for top dragstrip mph.

Rochester Q-jet WOT is the easiest to tune. All you have to do is change the metering rods. Fatter rods are leaner (less fuel)while thinner metering rods flow more fuel for a richer mixture.

The Edelbrock carb primary metering rods and springs are incredibly easy to change. Merely loosen the lid screw, slide the cover over and pull the rod and spring out. If you need to change primary or secondary jets, the lid must come off, which only takes a couple of minutes and you won’t spill any fuel.

The Q-jet uses one power-valve piston with a hanger to operate both metering rods in their respective primary jets. A stiffer power-valve spring pushes the metering rods out of the jets sooner (at higher vacuum levels). A softer power-valve spring uses less fuel by keeping the metering rods buried in the primary jets.

Setting idle mixture will change when you bolt on the air cleaner. If possible, set your idle-mixture ratios with the air cleaner in place. Holley and Demon carbs use a diaphragm-style primary power valve. The opening point is usually marked on the valve, such as a number like 65. This means that the valve opens (adding more fuel) at 6.5 inches of manifold vacuum. Using a lower-number power valve, like a 55 or 45 delays power-valve opening and could improve fuel mileage.

While the idle circuit is very basic, it has a huge impact on driveability. The idle-mixture screw controls the idle air/fuel mixture, but once the throttle is open the idle fuel restrictor combined with an idle air bleed determine the off-idle air/fuel ratio until the throttle blades open enough to create fuel flow out of the boosters and the main metering circuit.

Big cams with low idle manifold vacuum require opening to flow sufficient air to make the engine idle. If the throttle blades expose too much of the idle transfer slot (arrow) at curb idle. Then the usual fix is to drill two small holes (starting at around 3/32 inch) in the primary throttle blades on the idle-port slide of the blades. This shows the proper amount of the idle transfer slot exposed.

CC Tuning Tip
Poor ignition performance can be confusing when using the LM-1 air/fuel ratio meter. When a cylinder misfires, the air and fuel do not combust, meaning the oxygen in the air moves into the exhaust where the O2 sensor will immediately pick up the additional air and indicate a lean condition. Simply put, this means a dead cylinder or occasional misfire will show up on the innovate meter as a lean condition not a rich mixture as you might thing. Fix the misfiring cylinder(s) and the air/ fuel ration will read as a richer mixture.

Innovate Motorsports; Irvine, CA; 949/502-8400;