With all the interest in aftermarket electronic fuel injection, carburetors are often the forgotten form of induction. But when it comes to effectiveness per dollar spent, reliability, and ease of installation, carburetors are hard to beat.
One of the most innovative manufacturers in the carburetor business is Quick Fuel Technology (QFT). After opening their doors in 1998 the firm rapidly built a reputation for quality and performance in motorsports. Then in 2005 QFT introduced their very successful Super Street line of carburetors. Not a company to rest on its laurels, QFT recently introduced the RaceQ small flange drag race carburetor and the Hot Rod series, which is said to be the most affordable and tunable carburetor on the market today.
Carb 101 with QFT
The first thing required for a carburetor to function properly is atmospheric pressure. Pressure is the most important variable tied to a carburetor's performance, and without it one simply will not run! Most carburetors will have a vent tube that provides the carburetor with atmospheric pressure and forces the fuel to move through the metering passages as required based on engine demand.
A carburetor supplies fuel to the engine by something referred to as draw. Draw is essentially what the engine wants from the carburetor in terms of air and fuel. When an engine increases in rpm range, draw will increase. As draw increases a carburetor must react to properly mix the air and fuel together. Basically as more air passes through the carburetor, more fuel is drawn from the float bowl(s) and an atomized mixture is created.
If you've ever looked in a carburetor and seen the small diameter portion of the barrel, that's the venturi. The Italian physicist, Giovanni Venturi discovered that when air passes through a restriction two things happen: air speed increases and pressure decreases. Basically what happens in a carburetor is this: air passes through the restricted area(s), as there is atmospheric pressure acting on the fuel in the float bowl(s) the low pressure created draws atomized fuel into the airstream and sends it along to the intake runners.
It should be pointed out at low speeds and throttle settings airflow through the venturi is not sufficient to draw fuel from the main system, instead fuel is delivered through the idle ports below the throttle valves, which when almost closed create a venturi effect at the idle fuel port. As the throttle blades open vacuum at the idle ports decreases and fuel is now drawn from an intermediate, or transfer port. When the throttle blades open even further, airflow is sufficient to draw fuel from the main system.
Thinking out of the Carburetor Box
A question frequently asked of the tech team at QFT is how to properly install and set up a new carburetor.
While installing a carburetor is a simple procedure the QFT crew points out that there are a few crucial items needing special attention. A carburetor is designed to precisely meter fuel and air; if the sealing surface is damaged or obstructed the carburetor will essentially have an uncontrolled source of air—the dreaded vacuum leak!
A common mistake that can also lead to trouble is over tightening. Moving in a crisscross pattern will evenly distribute the torque of your wrench and ensure that the mounting flange of the carburetor is not damaged. Once the carburetor is properly mounted, take a few moments to move the throttle linkage. For your own safety it is imperative that the throttle returns to the closed position on its own and moves freely.
After the carburetor is mounted on the engine and everything else works properly the initial tune-up can be dialed in. Float level is very important to the overall performance; there are noticeable side effects if you deviate from this crucial step. Once the fuel bowls are filled and the engine starts, the float levels are set with the engine running at idle. Normally, a QFT carburetor will have the fuel level halfway up the sight window with 6.5-7 psi of fuel pressure.
The next step will be to set the idle mixture. Most four-barrel carburetors have adjustable idle mixture screws; some may have just two, while those for higher horsepower often have four, one for each barrel. The ideal method for setting idle mixture is with a manifold vacuum gauge. Setting the mixture screws to attain the highest vacuum reading is the standard method of adjustment, but that may not be the procedure for every engine combination. Performance engines often require more fuel at idle. Idle mixture is often a process that takes experimentation. Make mixture adjustments, slowly turning the screws half a turn at a time, keeping in mind that not all four screws need to be turned in or out the same amount.
Once the float level and idle mixture are set the next step is adjusting idle speed. Many carburetors have a transfer slot that aids the carburetor in its transition to wide open. If the throttle plate position is too far up (and into the transition slot) idle quality will be poor and the car might stumble off idle. The plate position can be corrected by adjusting the mixture screws (again something you will have to experiment with). The goal of a proper idle speed and mixture is to have an engine with a suitable idle while in neutral or in gear.
A carburetor that is adjusted properly during installation will give a solid baseline for fine-tuning to achieve maximum performance. When it comes to selecting jetting the most accurate means is with an air/fuel meter, however performance is a good indicator as well. Drag racers often use the best mph in a quarter-mile to determine proper jetting for best power. The procedure is to change jet size and record how it impacts mph. An increase indicates the engine needs more fuel; a decrease means the engine needs less fuel. Continue to go up or down with jetting until the mph drops below the peak, then simply return to that jetting combination that produced the best mph.