Changing Jets In Quick Fuel Carb: A Step-By-Step Guide

Carburettors are an effective, reliable, and easy-to-install induction system. Quick Fuel Technology (QFT) is one of the most innovative carburettor manufacturers, known for its quality and performance in motorsports. The company offers a range of carburettors, including the Super Street line, the RaceQ small flange drag race carburettor, and the Hot Rod series. One key aspect of carburettor function is atmospheric pressure, which is provided through a vent tube, forcing fuel to move through the metering passages based on engine demand. The draw, or the amount of air and fuel required by the engine, also plays a crucial role. Proper installation and setup of a carburettor are essential, paying attention to the sealing surface and torque distribution to avoid vacuum leaks. Fine-tuning involves adjusting the float level, idle mixture, and idle speed to achieve maximum performance. Jetting, or changing the jet size, can be done to optimise fuel delivery and engine performance.

The importance of atmospheric pressure

The carburetor is one of the oldest forms of induction for the internal combustion engine, and its performance is tied to the most important variable: pressure. Without atmospheric pressure, a carburetor will not run.

Atmospheric pressure is essential to the functioning of a carburetor as it forces fuel to move through the metering passages as required by the engine. This is achieved through a vent tube that provides the carburetor with atmospheric pressure. As the engine's rpm range increases, so does the draw, and the carburetor must react to properly mix air and fuel. The venturi, a small diameter portion of the carburetor barrel, is where air passes through a restriction, increasing airspeed and decreasing pressure. This low-pressure area draws atomized fuel into the airstream, which is then sent to the intake runners.

To address this, a pressure carburetor was developed, which operates on pressure alone, eliminating the reliance on gravity. This type of carburetor ensures reliable performance regardless of the aircraft's flight attitude. By understanding the critical role of atmospheric pressure, engineers have been able to design carburetors that meet the specific demands of different applications, such as high-performance racing or street cars.

How to drain the carb

To drain the carb, you'll need to first turn off the fuel valve. Next, locate the carburetor, which is usually found to the left of the recoil starter, between the air filter housing and the cylinder head cover. It resembles a grey cylinder with a metallic bowl at the bottom.

Once you've located the carburetor, prepare a gasoline storage container and put on some gloves to protect your skin from irritation. Identify the carburetor drain screw—this is usually an angled bolt at the bottom of the carburetor bowl or protruding from the side. Loosen the drain screw until gas starts flowing into the storage container. You may need to remove the screw entirely.

After the gas has fully drained, replace and tighten the drain screw, and store the gas appropriately. This process should leave your carburetor's fuel system in good condition.

If you're experiencing a lot of fuel leakage, you can try to identify and shut any valves along the fuel line to the gas tank. Alternatively, you can remove the fuel intake from the carburetor and plug it, or hold the end of the hose higher than the fuel level in the tank before draining the carburetor.

Tuning with air bleeds

The number of air bleeds a carburettor will have depends on the number of throttle bores and circuits it has. Most racing-style carburettors will have either 8 or 12 air bleeds, depending on whether they are two- or three-circuit. Each barrel will have one bleed per circuit.

If you want to richen up the idle, replace your idle air bleed with a smaller one. This will allow less air into the system, meaning more fuel and a richer idle. This will help get the mixture screws within the one- to two-turn range.

High-speed bleeds not only affect the mixture but also impact the fuel curve. For example, if the mixture gets richer at high rpm, a larger bleed will help.

Remember, it's important to take note of the stock specs so you can always return to where you started if needed.

The impact of weather

The weather can have a significant impact on carburetor jetting and engine performance. Changes in weather conditions can affect the air-fuel mixture and engine performance, requiring adjustments to the carburetor jets. For example, a drop in temperature between passes can result in a difference of a few thousandths on an E.T. (presumably Elapsed Time).

To compensate for weather changes, it is essential to establish a baseline for how the car performs under different weather conditions. This can be done by testing the car under various weather scenarios and recording critical data. For instance, you can record the temperature, whether it is sunny or cloudy, the E.T., MPH, and the jet used. By maintaining a log book of such data, you can better prepare for jetting adjustments at different tracks or locations based on the weather conditions.

Additionally, the weather can influence the ideal jet size for your carburetor. For instance, smaller jets with smaller holes should be used if your engine needs to run leaner, which is more suitable for warmer weather. Conversely, larger jets with bigger holes are preferable if your engine requires a richer mixture, which is often the case in colder weather.

The function of the venturi

The venturi is a crucial component of the carburettor, which is responsible for ensuring the engine receives the correct amount of fuel. When the throttle is opened, the carburettor jets control the amount of fuel that flows into the main circuit. Adjusting the jet size will either richen or lean out the fuel mixture for the entire fuel curve.

The size of the venturi is important for the effective operation of the carburettor. A small venturi may restrict high-speed engine operation, while a large venturi will not provide enough pressure differential for low-speed operation. Therefore, the production venturi size is usually a compromise to ensure adequate performance at both low and high speeds.

The venturi design can be tailored to specific conditions, with the curvature and length adjusted to provide the desired fuel flow. Most carburettors use a primary venturi and one or more boost venturis to further lower the pressure at the nozzle.

The carburettor's primary function is to mix intake air with hydrocarbon-based fuel, such as petrol or autogas (LPG). The basic operation of the carburettor has remained unchanged for decades, with the rudimentary parts, such as the body, bowls, and floats still in use today.

Frequently asked questions