Chevy Carburetor Tuning Tips - Dial In Your Carb

19 Tips From The Foremost Carb-Tuning Experts Reveal Their Secrets

Stephen Kim Oct 1, 2008 0 Comment(s)
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Rome wasn't built in a day, and neither were carburetors. Just like the ancient Roman aqueducts achieved a technically demanding task using archaic resources, the carburetor was designed to perform complex fueling duties without the assistance of fancy microprocessors and electrical sensors. So while the EFI punks with laptops may mock carbs as dinosaur technology, getting the same job done without the luxury of high-tech electronic doodads is, arguably, an even more impressive feat.

Granted, the inner workings of a carburetor are incredibly complex, but don't sweat it if the other guys in your car club are the real carb gurus. As long as you can identify the basic components of a carburetor and are familiar with the functions they perform-like the vast majority of enthusiasts, in other words-you'll find the information divulged in this story completely palatable. Rather than attempting to dissect every nook and cranny of a carburetor, we'll instead focus on how to cure the most common tuning ailments and how to avoid the most common goofs. If big boost, big spray, or big cams are your thing, we've got that covered as well. Furthermore, since carbs are often erroneously blamed for a multitude of engine woes, we'll explain how to isolate ignition and fuel system issues from actual carb problems.

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For assistance, we turned to an all-star panel of distinguished carb experts that includes Jay McFarland of Holley, Adam Campbell of Barry Grant, Patrick James of Pro Systems, Marty Brown of Quick Fuel Technology, Sean Murphy of Sean Murphy Induction, Bob Vrbancic of The Carb Shop, and Kevin Van Noy of Carburetor Solutions Unlimited. Here's what they had to say to help ensure that your carb performs like the atomizing machine it was built to be.

Problem: No Matter What I Try, My Motor Runs Rich.
The obvious place to start is by checking to make sure that the float bowl level is set properly. Most carb manufacturers and tuners suggest setting the float level at roughly 25 percent of full capacity, or filling the bowl up to the bottom of the sight glass. If that checks out, the jets aren't ridiculously large, and the power valve is properly sized, a probable culprit is the ignition system. "Make sure you're getting a solid 12 volts to the distributor under all operating conditions," advises Campbell. "Low voltage will make it seem like the motor is running rich. What throws people off is that a subpar ignition system may deliver 12 volts at idle, but drops down to 6 volts or less under load. Another common problem is that the balancer may have spun on the crank snout, or the motor may have the incorrect timing pointer for the balancer, which makes it impossible to properly set the timing."

Problem: My Car Lays Over At High Rpm, And Up-Jetting Does Nothing.
If dropping in larger jets has no effect on a power drop at high rpm, the motor probably has a bigger appetite than the fuel system can handle. Bigger fuel lines and a high-flow pump should eliminate the problem. "People often get so excited about building a new motor that they overlook the fuel system or never even bother to check what size fuel lines they have after they buy a car," says Campbell. "If you add another 200 hp but don't upgrade your fuel system, no matter how you tune the carb, the motor isn't going to make a lot of power."

Problem: My Idle Circuit Must Be Messed Up, Because The Idle Is Way Too Rich.
When a motor runs rich at idle even with the right size power valve and the adjustment screws turned all the way in, it may be time to bust out the fuel pressure gauge. "The biggest problem we encounter in our shop is cars with excessive fuel pressure," explains Murphy. "Carbs love volume, but hate excessive pressure. The most pressure you want to run is 7 psi, and 10 psi is excessive. Design a fuel system with minimum pressure and maximum volume." Things get a bit trickier with a big cam, as low vacuum requires opening up the throttle blades to maintain a steady idle. This exposes more of the transfer slot, resulting in a rich condition. McFarland advises that no more than 0.025 to 0.030 inch of the transfer slot should be exposed past the throttle plate at idle. To compensate, the secondaries can be cracked open slightly or two small holes can be drilled on the primary throttle blades. Start at 1/16 inch, then drill progressively larger holes until the desired idle speed is reached. Barry Grant even offers carbs with holes already in the baseplate, eliminating the need to drill.

Problem: My Motor Hesitates, Backfires, And Won't Hold A Steady Idle.
Many hot rodders crank up the idle circuit to richen up the air/fuel mixture in this situation, but that's the last thing you want to do. Chances are these symptoms are caused by too much cam and not enough timing. "Big cams and compression don't produce much cylinder pressure at idle, so there's no way a motor will run properly with only 8 degrees of initial timing," says Vrbancic. "You can't expect a performance motor to run well with the same timing as with a stock camshaft. Simply turning the timing up to 15 degrees to compensate for the low vacuum usually fixes the problem entirely."

Problem: I Spec'd My Carb Using A Formula Out Of A Book, But I Think It's The Wrong Size.
Proper sizing is half the battle when dialing in a carburetor, and our expert panel unanimously agrees that the formulas found online and in books border on useless. "If you follow those formulas, everyone in Pro Stock is running carbs that are too small," quips James. "A big-inch 600hp motor that doesn't turn a lot of rpm requires a vastly different carb than a small-inch 600hp screamer even though they both make the same power. Make your decision based on what size carb others have used successfully in applications similar to yours, and keep extensive records of what works and what doesn't." Likewise, according to Campbell, sizing a carb based on cam duration at 0.050 can greatly simplify the process. "There are so many variables when sizing a carb, such as the power level, displacement, rpm range, transmission type, converter stall speed, vehicle weight, and so on," he says. "Since cam selection also accounts for all these variables, sizing a carb based on cam duration is a good approach."

Problem: I Just Installed A Centrifugal Blower, And Now My Carb Can't Keep Up.
Maintaining a consistent fuel curve with a blow-through supercharger arrangement is particularly difficult due to the rapidly changing fuel demands of a motor that cycles in and out of boost. The key is to resist the urge to plop a big carb on the intake to ensure good drivability off boost, and to modify the carb's internals for additional fuel flow under boost at high rpm. According the Brown, a 750-cfm carb can easily support 1,000 hp in a blow-through application. In addition to installing annular boosters that increase fuel flow at low rpm, just about all the internal fuel passages must be increased in size to support additional fuel volume. "In our blow-through carbs, we make the main wells, fuel passages, and needle-and-seat assemblies larger," he explains. "The fuel system needs to be addressed as well, since high volume is a good thing but high pressure can aerate the lines." Additionally, machining lip seals into the baseplate prevents pressurized air from escaping in high-boost applications. Van Noy has been at the forefront of blow-through carbs for years, and has a few extra tricks up his sleeve. "We make our own boost-referenced power valves that only open up until you hit a predetermined boost level and a custom booster that enhances signal at low rpm," he says.

Problem: I Want To Impress My Buddies With A Custom Carb, But I'm Not Sure If I Need One.
Factory carb calibrations work great for the vast majority of applications, but the bigger the cam, the more you can benefit from a custom-tuned carb. "All universal carbs are just that, universal," explains McFarland. "Although we strive to get the best calibration as possible for most engines, and they work very well out-of-the-box for most applications, there is always a little more to be had by tuning a carb specifically to your engine's needs." Once duration numbers are in the mid-240- to 250-degree range, engines become more finicky and often require constant tuning. "You can expect to pick up one- to three-tenths in the quarter with a custom carb in a typical street/'strip car, but an even bigger benefit is the improved idle quality, throttle response, reliability, and consistency," explains Murphy. "Once a custom carb is set up, all you have to adjust are the jets."

Problem: Whenever I Pop My Hood, People Laugh At My Vacuum-Secondary Carb.
Double-pumpers look tough and just about every race car ever built has one, but they aren't always the best choice for most street applications. If torque converter stall speed and lockup aren't properly matched to cam duration, mechanical secondaries will often open too soon, killing precious vacuum and air speed when the motor needs it the most. "For most street cars and even many street/'strip applications, a vacuum-secondary carburetor works best," says McFarland. "They work very well on midweight or heavyweight cars with an automatic transmission. Since they operate by sensing engine load and only open when the extra fuel is needed, they are more forgiving than mechanical secondary carbs. The mechanical secondary carburetor is best suited for lighter cars with radical camshafts, short gears, and manual transmissions, or for full-blown race cars."

Problem: My Downleg Boosters Look So Antiquated. Should I Upgrade To Annulars?
Annular boosters provide superior low- and midrange drivability compared to straight and downleg boosters due to their improved atomization of fuel. Likewise, since they're larger and occupy more space in the venturi, annular boosters also amplify carb signal. While those characteristics are desirable in low-rpm street motors, they result in a rich condition that's difficult to control at the top of the powerband in high-rpm 'strip motors. "Annular boosters become restrictive at high rpm, so you're better off with a downleg booster in a street/'strip or race application," says Vrbancic. "On the other hand, annulars work great for most low-rpm street cars." Moreover, booster selection needn't be an either/or proposition. Using annular boosters on the primary side and downleg or straight boosters on the secondaries can yield excellent low-speed drivability in addition to precise control of the fuel curve at high rpm.

Problem: My Car Hesitates When It Dead-Hooks Out Of The Hole.
If your car hooks up hard enough to uncover the jet at launch, setting the float level higher-about 3/4 up the sight glass-is a quick fix. An even better solution is installing a set of jet extensions. "Once you hit the 1.5-second, 60-foot range, uncovering the jets becomes very common," says James. "That said, we recommend extensions in all drag racing applications, because regardless of your 60-foot times, you never know how a car is going to pitch at launch."

Problem: My Throttle Plates Are Almost Completely Closed, But My Motor Idles Too High.
In this scenario, air is entering the motor from a source other than the carburetor, which suggests a leak at the intake manifold. "Overtightening the manifold bolts, gasket misalignment, and using the wrong gaskets can all cause leaks," says Brown. "It's a very common problem that results in low manifold vacuum, a high idle speed, and lots of fumes and hydrocarbons out the tailpipe." Murphy estimates that 60 percent of all the cars he works on have a manifold leak. "A small leak may only kill one cylinder, which makes the motor idle rough, but everything smoothes out as the increase in rpm and air velocity moves enough fuel to cover it up," he says.

Problem: How Do I Know What Power Valve To Use?
With motors that produce at least 12 inches of vacuum, a 6.5 power valve will work fine. If your motor pulls less than 12 inches of vacuum, the vacuum reading should be divided in half. For instance, if the vacuum gauge reads 9 inches, a 4.5 power valve is recommended. "A lot of time people change the power valve without knowing what it's doing," explains Vrbancic. "People often try to adjust the idle mixture screws when the actual source of excess fuel is a mismatched power valve that opens too soon. If the vacuum is jumping all over the place on the gauge due to a long-duration cam, keep the power valve [number] farther away from the vacuum signal. For example, with 10 inches of vacuum, that means going with a 4.5 power valve instead of a 5.5."

Problem: Does It Matter That My Car Hasn't Got Much Hood Clearance?
Adequate hood clearance is essential to healthy carb operation. Holley recommends at least 3/4 inch of clearance between the carb and the hood or between the air cleaner and the top of the vent tubes. "Without adequate clearance, the fuel bowls will not be able to vent, which will cause pressure buildup in the fuel bowls," explains McFarland. "Instead of the engine drawing in fuel, this pressure will push fuel out of the boosters, resulting in an extremely rich condition or even engine stall."

Problem: My Motor Feels Lazy After Each Shift, Then Picks Up Again.
To prevent sluggish shift recovery and the resulting slower e.t.'s, properly sizing the fuel channel passages in the metering blocks is essential. If the channel area is too large on a high-horsepower drag car, the jolt following each shift will cause a lean hesitation. "Billet metering blocks do not respond as fast to shift recovery as cast blocks," explains James. "That's because their fuel channels are cut with an end mill, and they end up being too large. The smaller you can keep the channel area, the faster it can respond after shifts, and reducing the cross-section of the channels by filling them with epoxy improves shift recovery dramatically. The problem is far less pronounced with cast blocks, since their fuel channels are smaller to begin with."

Problem: My Carb Loses Signal When I'm On The Bottle.
For most street applications where nitrous is limited to 150-200 hp, an out-of-the-box carb will work just fine. However, there's only so much volume beneath the plenum, and once nitrous dosages eclipse the 300-400hp mark, the nitrous displaces lots of air, in turn reducing carb signal. A simple fix is dropping down the size of the carb a tad, but every application is different. "Sometimes we'll reduce the size of the venturi, in other instances we'll increase the size of the booster, and some motors require doing both," explains Vrbancic. "It really depends on the specific engine combination, but the main goal is preventing a lean condition and keeping the signal strength up."

Problem: I Don't Know How To Adjust My Four-Corner Idle System.
A trained ear can adjust the idle air/fuel mixture screws by sound, but there's nothing wrong with relying on a vacuum gauge. "What you're trying to do is look for the highest point of vacuum as you turn the screws, inward to lean the mixture and outward to richen it up," explains Campbell. "Start at driver-side front and go around clockwise, then go around a second time to fine-set those screws. Since the front screws also feed into the idle circuit, they should be out even with or slightly more than the back screws. You want the screws to be as even as possible, within 1/2 to 1/4 turn of each other."

Problem: I Have To Starve For A Week In Order To Afford The Gas For Cruise Night.
Although carb tuning can't make up for fundamentally poor engine and driveline design, it can yield gains of a couple of mpg. "Run as small of an accelerator pump as possible so you're not squirting any more raw, unatomized fuel into the motor than you have to," advises Murphy. "Outside of drag motors, most cars don't need that much fuel off-idle. If you have a wideband O2 sensor, you can tune your carb to run as high as a 16:1 air/fuel ratio at cruise without any detrimental drivability effects. The more vacuum advance and initial timing you run, the more vacuum the motor will produce, so the less throttle you'll need. With these simple tricks, we've had customers pick up 2 to 3 mpg." Moreover, since Quadrajets and Edelbrock/Carter carbs utilize a tapered metering rod-that can precisely meter fuel at part throttle-in addition to jets, they typically offer better gas mileage than modular Holley designs.

Problem: I Installed A Big Cam, And Now My Motor Hesitates Off-Idle.
This is a very common gremlin associated with long-duration camshafts, but with diligent tuning it can be eliminated. The first step is to make sure the float level is neither too high nor too low, and that the idle mixture screws are set correctly. Likewise, the throttle blades should be angled properly in relation to the transfer slot, and the power valve needs to be matched to manifold vacuum. "If all of these preliminary diagnostics check out and the motor has sufficient initial timing advance, then it's time to modify the accelerator pump with a different cam and a larger squirter," explains Brown. "The idea is to introduce more fuel in a burnable fashion to go along with the big slug of air you just introduced to the motor after whacking the throttle. A four-hole spacer can help amplify signal to the carb, and shorter rearend gears can help as well."

Problem: When The Weather Heats Up, My Car Runs Like A Turd At The Track.
While no amount of tuning can make up for the thinner air that summer brings, rejetting to prevailing track conditions minimizes additional power losses that can result from running rich. The great thing about track tuning is that you're essentially on a dyno. "Since trap speed is an indication of power, just pay attention to what the mph does and don't worry about the e.t.," explains Van Noy. "Different wideband O2 sensors read differently, so they can't be an end-all tool. Tuning based on mph works every time. After you've achieved the ideal jetting based on mph, pull the spark plugs and they'll give you an idea of what to shoot for."


Quick Fuel Technology
Bowling Green, KY 42101
The Carb Shop
Ontario, CA
Carburetor Solutions Unlimited
Fontana, CA 92336
Sean Murphy Induction
Huntington Beach, CA 92647
Pro Systems
Spring Lake, MI 49456


How To
Read about Carburetor tuning tips including power valves and centrifugal blowers.
Stephen Kim Oct 1, 2008


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