What is a Float-Type Carburetor System?

Posted on November 17, 2021 Jerold Perkins Aerospace

A float-type carburetor consists of six subsystems that control the amount of fuel that is discharged with regard to the flow of air being delivered to the engine cylinders. These systems work in tandem to provide the engine with a fixed quantity of fuel flow during all engine operating ranges. The six subsystems within this carburetor include a float chamber mechanism, main metering system, idling system, mixture control system, accelerating system, and economizer system.

Float Chamber Mechanism
A float chamber is typically positioned between the fuel supply and the main metering system of the carburetor. Shaped like a bowl, the float chamber serves as a reservoir for the incoming fuel. This chamber delivers a constant quantity of fuel to the main discharge nozzle which is usually about ?” below the holes in the main discharge nozzle.
The level of fuel is controlled by a float-operated needle valve and a seat. If there is no fuel in the float chamber, the float drops to the bottom of the chamber and enables the needle valve to open wide. As the fuel is delivered to the supply line, the float rises and closes the needle when the fuel reaches the desired level. The valve takes an intermediate position so that it can continue supplying a consistent amount of fuel. 
Main Metering System
The main metering system is tasked with supplying fuel to the engine at all speeds above idling and consists of a venturi, main discharge nozzle, fuel metering unit, passage leading to the idling system, and throttle valve.
The venturi is responsible for balancing the fuel-air mixture, decreasing the pressure at the discharge nozzle, and limiting the airflow at full throttle. The discharge nozzle is located in the carburetor barrels so that one end is open toward the narrowest section of the venturi. The fuel metering unit is positioned in the fuel passage between the float chamber and the discharge nozzle limits the flow of fuel when the throttle valve is open.
Idling System
At idling speeds, the throttle valve is closed or marginally open. A low air velocity through the venturi does not allow the discharge nozzle to deliver an ample amount of fuel. However, near the engine side of the throttle valve exists an idling jet which serves as a fuel passageway, allowing the engine to idle.
Mixture Control System
As altitude increases, atmospheric air becomes less dense. Above 18,000 feet, the air is only half as dense as it is at sea level. With this in mind, it can be concluded that an engine cylinder full of air at 18,000 feet contains only half as much oxygen as a cylinder full of air at sea level. While the low-pressure area created by the venturi relies on air velocity rather than air density, the venturi draws the same volume of fuel through the discharge nozzle whether it is at a high or low altitude.
As a result, the fuel-air mixture becomes richer as altitude increases, necessitating a manual or automatic mixture control mechanism. One of two devices is typically implemented in a float-type carburetor to control the fuel-air mixture: the first is a needle type, and the second is a back-suction type.
Accelerating System
If the throttle valve is opened too quickly, the fuel-air mixture leans out which can cause the engine to accelerate slowly or stall as it attempts to accelerate. To remedy this, the carburetor is equipped with an accelerating pump, or piston pump. This pump is operated by the connection between the throttle control and a passageway opening into the main metering system or the carburetor barrel near the venturi.
When the throttle is closed, the piston pulls back and fills the cylinder with fuel. If the piston is moved forward slowly, the fuel makes its way into the float chamber. If the piston is pushed too quickly, it sprays fuel into the venturi and enriches the mixture.
Economizer System
For an engine to achieve maximum power at full throttle, the fuel mixture needs to be richer than what is needed for cruise speeds. The extra fuel is utilized to cool the engine combustion chambers in order to prevent detonation. An economizer is a valve that remains closed at throttle setting below 60-70% of rated power. An economizer system typically consists of a needle valve that opens when the throttle valve reaches a wide-open position, allowing fuel to flow through it. There are two types that are often used with this type of carburetor: a pressure-operated economizer system and back-suction economizer system.
If you find yourself in need of parts or components for a float-type carburetor, or another carburetor variation, rely on Aerospace Exchange for all your operational needs. With an ever-expanding inventory containing billions of in-demand items sourced from trusted global manufacturers, you are bound to find what you need with ease. Kick off the procurement process with a competitive quote for your comparisons, and a dedicated representative will reach out to you in as little as 15 minutes.

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