For the uninitiated, carburetor tuning can seem like some sort of dark art—an enigmatic skill passed down to a select few through underground channels. Although these time-honored devices are an impressive feat of mechanical engineering, carburetors aren’t the inscrutable mystery they may appear to be at first glance.
“As long as the carburetor is correctly sized for the application, our calibrations tend to be very close right out of the box,” explains Tom Kise, Corporate Technical Training Manager for Holley. But that doesn’t mean you can just blindly fumble your way through the tuning process and expect to end up with a carb that’s operating efficiently.
“The idle mixture is going to give you a baseline foundation, but every vehicle is different. You can have a vehicle that’s properly dialed in—idle mixture, jetting, all of it—and simple things like changes in elevation will have a significant effect on how the carburetor operates. At the end of the day, you have to tune based on what that particular engine wants. There’s no magic setting out there that works across the board.”
With that in mind, we cornered Kise and managed to get him to reveal the secrets to successful carburetor setup and basic tuning. He was quick to point out that while manufacturers design their carburetors differently, the fundamentals tend to be universal.
“All carburetors follow the same core principles. One design might not look the same as another, but you’ll find the same basic adjustments on both of them—they may just be located in different spots.”
GETTING STARTED
Along with a set of hand tools, Kise recommends having fuel pump and vacuum pressure testing gauges on hand for this task. Also, keep in mind that this work needs to be done on level ground, and the engine should be brought to operating temperature before making adjustments.
“The choke adjustment is the one exception to that rule,” Kise says. “The choke is used for cold starts and warm-up operation, so that needs to be adjusted while the engine is cold. That should be done after you’ve completed the other initial tuning tweaks. You can look at things like the choke and acceleration enrichment as modifiers; they’re modifying behavior off of the base parameters. Idle, jetting, timing—all of that stuff needs to be properly adjusted before you tune anything that modifies the carburetor’s behavior off of that base parameter. Choke, fast idle, and acceleration enrichment really should be among the last things that you fine-tune.”
With any new setup, the first element that should be checked and adjusted, if needed, is the fuel pressure and float level.
“If you’re using an adjustable regulator, connect your gauge and adjust the regulator while flowing fuel,” Kise says. “The sweet spot for the fuel pressure level is around 6 to 6.5 psi. It should be no less than 5 psi and no more than 7 psi.”
For externally adjusted needles and seats, the float level should be adjusted by loosening the lock screw and adjusting the nut. Turning the nut clockwise will lower the float level, and turning it counterclockwise will raise the level. “The sight plug bowls should have the level adjusted to just below the hole—fuel should only trickle out when the fender is gently rocked,” he explains. “Sight window bowls should be at the middle or just below mid-level.”
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SETTING THE IDLE MIXTURE
Kise tells us that the idle adjustments are the foundation for all the other elements of carburetor tuning. “When the engine is idling, the mixture screws should have complete control over the fuel entering the engine and how it runs. If the mixture screws have no adjustment or control, fuel is getting into the engine from somewhere other than the idle circuit. With the screws turned all the way in, the engine should stall.”
He also points out that new Holley carburetors are preset to a base parameter during factory calibration, and that calibration should put you in the ballpark.
If the carburetor you’re working with isn’t new, turn the mixture screws inward until they’re lightly seated, then back the screws out about one to one and a half turns. Then, with the engine up to operating temperature and idling, and with the vacuum gauge connected to full manifold vacuum, adjust one mixture screw 1/8th of a turn at a time with the goal of increasing the vacuum gauge reading. “Any adjustment made to one screw that yields increased vacuum should be matched on all other mixture screws,” he says.
If the carburetor you’re working on is properly sized for the application, Kise suggests starting with the stock jetting and going from there.
“There isn’t a ‘one size fits all’ option for jetting. Each engine is different, and jetting adjustments often come down to some trial and error. The engine should be run under a load and spark plug color should be noted—it’s a good idea to have a few sets of spark plugs on hand during initial tuning.” For street-driven applications, Kise recommends tuning the primary side first, as part-throttle acceleration and cruising are the conditions that the engine will most commonly see.
“Secondaries can then be tuned by running the engine at wide-open throttle for at least ¼ mile and shutting down under throttle to get the most accurate plug reading,” he says. For the sake of safety, this should be performed at a track or on a loadable chassis dyno.
“The base calibrations for carburetors are adjusted for sea level, and you need to keep that in mind here, too. So, if you’ve just bought a new carb and you’re at 8,000 feet of elevation in Colorado, that base calibration is going to be too rich for that elevation. The general rule of thumb is that for every 2,000 feet of elevation increase over sea level, you drop one jet size.”
Once you’ve got the mixture and jetting sorted out, Kise recommends moving on to power valve adjustments. Specific to Holley carburetors, the power valve is a simple diaphragm that is typically located in the primary metering block, and its purpose is to improve part-throttle drivability, air-fuel ratio, and fuel mileage. You can check out our power valve tuning guide for a deep dive into the finer points of adjustment, but in the meantime, Kise offers some general best practices.
“The power valve should be sized after peak manifold vacuum at idle has been reached. In most cases, sizing should be half of the idle vacuum when making 12” Hg or less. If it’s greater than 12” Hg, you can use a 6.5.”
Once the fuel pressure, float level, jetting, and (if applicable) the power valve have been sorted, you can then move on to acceleration tuning.
“These should be done before upping the nozzle size or changing pump cams. Increasing nozzle size to cover up another issue is only a Band-Aid, and it may create another problem elsewhere once the actual issue is addressed.
It is also critical that the accelerator pump arm is properly adjusted. It should be set to zero lash—there should be no gap or preload between the pump arm and lever. At wide open throttle, you should be able to fit a minimum of a .015” feeler gauge between the pump arm and the lever without it compressing the override spring.”
If all the other adjustments are dialed in and the engine still has a hesitation, that’s when Kise recommends increasing the nozzle size. “Increase it by one size for a hesitation that’s one second in duration or less, or two sizes for a hesitation longer than one second.”
When it comes to testing and troubleshooting, he cites listening to the engine and reading spark plugs as the best ways to get a read on the health of the tune. “Although it’s application- and fuel-specific, the color on the plug can tell you a lot about what’s going on. And some issues can provide clues about what part of the tune is off. Lean jetting, for example, can cause tip-in hesitation and part-throttle surges. If the tune is excessively rich, you’re likely to see things like fouled plugs, black smoke, and sluggish response.”
It’s also important to eliminate outside variables from the troubleshooting process whenever possible. “Your combination can be a factor as well,” he adds. “Sometimes the carburetor isn’t the issue. If you’ve got a really aggressive camshaft and a big intake but you don’t have the right gearing—or you have a torque converter with the wrong stall speed—and you’re loading the engine at too low of an RPM, no amount of carburetor tuning will correct the hesitation that will cause.”
For those who want to dive deeper into carburetor tuning, he notes that Holley offers a comprehensive two-day training course at its Bowling Green, Kentucky-based facility.
“We’ve talked about the key fundamentals of carburetor tuning here, but of course, it goes much deeper than that. In our class, folks can bring in their own Holley carburetor to work on, or we can give them a loaner carburetor to use for the course. In class, we’ll tear the entire carburetor down and go through every circuit of it in-depth, discussing functionality, tuning, and how to troubleshoot issues. It takes a lot of the mystery out of how these things work.”
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