Four-Barrel-Carburetion Tech Tips - Carbin' Out

Technically, the system is dependent on pressure, both inside the engine and inside the carburetor. An ultra-simple example can be found by sucking a drink through a straw. As the straw sits idle in the beverage, atmospheric pressure is placed on the drink and inside the straw. This pressure is generally 14.7 psi at sea level. We call it the "atmosphere." When you suck on the straw, you create a low-pressure area in the tube. Since the pressure being applied to the beverage becomes greater than the low pressure in the straw, the beverage rushes in to fill the void.

In the world of carburetion, this draw is called "the signal," and a strong signal is important. From the straw example, you can also see how sizing is critical. Here's why: If you increased the diameter of the straw from a standard size straw with approximately a 1/4-inch diameter, to an unrealistic 2-inch-wide straw, you would now have the capability of flowing plenty of fluid, however, you probably wouldn't be able to generate enough of a draw with your lungs to get the fluid to move. Scientifically speaking, you wouldn't be able to create the necessary pressure drop to allow fluid to move.

Conversely, if the straw size was reduced to 1/8 inch in diameter, you wouldn't get enough beverage to match the capacity, or displacement of air, when you inhaled. In other words, you need a straw that's matched to your lungs, and you need a carburetor that's matched to the displacement of your engine. Furthermore, if you have a carb that's too big, the airflow will be slow and the signal weak, and if the carb is too small, the airflow and draw would be good at low rpm, but it would become restrictive and limit high-rpm power and peak efficiency.

Supreme Circuitry
It is necessary for carburetors to have a number of circuits to handle the airflow and fueling demands during all facets of operation. The fuel bowl serves as the fuel reservoir and it holds the fuel supply, just prior to the fuel being used by the engine. To maintain the proper level in the bowl, a float is employed. When fuel is drawn from the bowl, the fuel level sinks and the float falls. The float is connected to a needle valve, which opens to supply more fuel. When the float rises, the valve seats and the fuel flow stops. It's important to have the float level set correctly in order for the carburetor to function properly.

"Getting the float level set correctly is critical to good operation, but many people have a problem with this and with fuel pressure," says Patrick James of Pro Systems. "When the fuel pressure is set too high, you will have to run the floats too low. And when it drops, fuel rushes in at a pressure that's too high and the fuel can actually foam up. Typical fuel pressure should be no greater than 7-1/4 psi."

Marvin Benoit of Quick Fuel Technology agrees. "One of the biggest tuning issues revolves around float level and having incorrect fuel pressure, which causes you to run the floats too low," he says. "In this situation, the bowl has less fuel to begin with, so it empties quicker, and with that the boosters won't react as quickly either because the fuel level in the main well is not high enough. So, the fuel can't keep up. People think higher pressure means more fuel, but it doesn't work that way." And with the fuel pressure too high, fuel will rush in so quickly that it can foam up, causing your lean condition to worsen.

Most street-driven vehicles will have a carburetor that's equipped with a choke system, and this enables the carburetor to supply a richer air/fuel mixture to assist starting when the engine is cold. The choke is usually a butterfly plate located at the top of the air horn. It can be manually or electrically operated, but the air horn and choke plate restricts the airflow so it is removed on most racing carburetors. On some models, the entire air horn is removed, revealing a smooth entry into each venturi.

Once the engine is running and an idle has been established, the idle circuit will be operating. One inherent problem is that, during idle, the action of the pistons generally isn't strong enough to create the proper draw on the venturi, and this can result in a lean condition, poor idle, and at times, a stumble during tip-in or launch in racing applications. To remedy this, the idle circuit should provide the necessary fuel flow for idle and off-idle acceleration. Rather than rely on the boosters, each idle circuit has a series of air bleeds, mixture screws, and discharge ports to control the air/fuel mix at idle.

"Idle quality is one of the biggest things to consider," James says. "If the engine is too rich at idle, it could take a good portion of the track before it clears up. The plugs may look good at the end of a run, even lean, but you wouldn't know it. The engine could be at 11:1 off idle and 14:1 at the finish line, and the plugs would look like the engine is running at 13:1. Being rich at idle could be caused by timing that retarded too much, air bleeds that are too small, the wrong heat range plugs, intake runners that are too large, even the wrong fuel. If the engine is too lean at idle, the engine won't respond. It may bog or detonate."