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.
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.
SOLUTION: 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.
Solution: 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."
In essence, carburetion is...
In essence, carburetion is nothing more than a low-psi mechanical fuel injection system. The pressure differential between ambient air and the low pressure area inside the venturi pushes fuel out of the fuel bowl, through the emulsion tube, and into the booster. Rather than being pulled into the motor through engine vacuum, fuel is actually pushed through the carb through pressure differential.
Giovanni Battista Venturi,...
Giovanni Battista Venturi, an 18th century Italian physicist, didn't give a rip about carburetors, but his discovery is what makes carburetion possible. He learned that when fluid flows through a constricted space, it creates a low-pressure, high-velocity area at the constriction. This is precisely what happens in a carburetor's venturi, allowing air and fuel to be drawn into the motor.