Four-Barrel-Carburetion Tech Tips - Carbin' Out

Connect the fuel lines, and you'll be ready to start the engine. When you do, immediately check for fuel and vacuum leaks and set the engine idle speed (rpm). If the engine has a vacuum leak, it's likely you'll hear a high-pitched whistle. Fuel leaks may not be visible, so feel around the bowls and fittings.

There are a few adjustments you can make to the carburetor to improve driveability, however any serious performance tuning should be done on a dyno or at the track. Street engines can be tuned, using a vacuum gauge, by reading your spark plugs or by using EGT or O2 sensing data. "I suggest learning to read plugs," James says. "Every engine is different and will want a slightly different tune, but you should look for a sharp carbon ring at the base of the porcelain (on the plugs) and a blueish line in the center of the ground strap-that is a good tarting point. We sell a video for $20 that covers this topic."

To a degree, carburetors are self-metering, but they only know air mass, and therefore supply fuel based on the mass of air flowing through the venturi, regardless of the density of the oxygen in the mass of air. So, if the weather conditions worsen, the carb may not flow less air, but the quality of the air may require that less fuel be added to maintain the best possible air/fuel ratio. "Carburetors respond to changes in air mass, not oxygen density. A general rule is to go one jet size for 1,500 feet of altitude." But James cautioned that this is not a concrete rule; it depends on what the engine wants.

Excess fuel, as most of us know, creates a rich condition, which can be noted by fouled spark plugs and black smoke from the exhaust. A lean condition is more dangerous (to the engine) because it leads to engine damaging detonation. The correct air/fuel ratio will net the most efficiency and the most power.

According to Doug Schriefer of Demon Carburetion, there is one common mistake people make when selecting a carburetor. "The first mistake is size; most people buy a carb that is too large for their engine. They'll look at a dyno sheet or a story and see the larger carburetor makes another 5 hp over the smaller carburetor, but they don't realize that the point at which it made more power is not within their useable powerband."

In drag racing, the most common carburetor problem is seen as a bog off the line. Miele explains, "A bog is probably caused by fuel starvation. To cure this, keep the rear float slightly higher than normal. Use a clear sight plug in the side of the bowl, and get the fuel near the top. Naturally, you'll have to fine-tune to find the right spot. I also run 7-8 psi of fuel pressure, with the number 10 line to the regulator-that seems to be the norm."

Victor Moore of Demon adds, "The first thing is to realize that carburetors are extremely simple. All they do is mix the air and fuel going into the engine. If you have the proper carburetor, and you're having a problem with it, that means you either have too much fuel or not enough, so figure out when the problem is happening. At idle, during acceleration, steady speed, and so on, add fuel at that point, and if it is a fuel problem, it will get better or worse and then let you know where to go.

"For example, when tuning the idle circuit, if you're not sure if the engine is too rich or too lean, press on the accelerator pump arms without moving the throttle linkage. This will add fuel without adding any air and let you know if the engine wants more fuel or not. If the idle speed increases, the idle circuits are being deprived and need to be enriched; conversely, if idle speed falters, the idle circuits need less fuel.

"Also, modular carburetors will require a minimum amount of maintenance to work optimally, so make sure your bowl screws are tight, and that the air bleeds in the top of the carburetor are clean and free from any debris. A loose bowl screw can make a carburetor run poorly, and dirt can cause major problems, too."

Championship-winning drag racer Charlie Booze noted that he has a six-step procedure any time the carb is removed. First, he checks the throttle linkage for binding. Then he checks the baseplate screws and the bowl screws, as these can vibrate loose, especially when solid engine mounts are used. Third, he cautions to not overtighten the carb when bolting it to the manifold. He then checks the throttle once more, after the carb has been installed, and he will prime the carb using the electric pump. Then he'll squirt fuel using the throttle and shut off the pump. Next, he will start the engine and then turn on the pump, which he says prevents the pump from overfilling the bowls due to the needle and seat sticking.