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Tech Talk | The float carburettor

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It is one of the most important internal components of a kart, affecting both engine performance and wear. It regulates the air-to-fuel ratio and the amount of this mix to flow into the engine, based on how much it accelerates. It therefore defines the combustion specifications. The float carburettor (also called “needle” carburettor) is mainly used in classes with gears, but not only. Let’s find out how it works

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The float carburettor is a component formed by a main body, generally made of cast iron, and by a lower float bowl screwed into the main body. The key elements are the diffuser (or Venturi) or the main duct for the flow of air that comes from the filter and flows towards the engine. It first has a concave and then convex shape and the minimum diameter strongly characterizes the carburettor and its performance in relation to the engine, since it is proportional to the displacement of the engine with which the carburettor is combined and to the power that the engine itself can deliver. In the middle of the diffuser is the fuel valve, or guillotine, which can have a flat or “piston” shape. It works thanks to the vacuum that is created in the diffuser, whose minimum diameter is expressed in mm. The incoming air in this duct, pulled by the vacuum in the pump casing and the pulsations of the piston, accelerates as the duct narrows, and then slows down again in the divergent section. The more the speed of a fluid increases, the lower the pressure in the fluid.

Thus, the air pressure is minimal at the narrowest point of the Venturi. It is precisely in the vicinity of this section that the carburettor duct has holes, or nozzles, through which it sucks petrol mixed with lubricating oil, due to the effect of the vacuum. The fuel introduced into the air flow tends to evaporate due to the low pressure, speed and turbulence of the flow. Externally, the carburettor has a starting lever connected to the starter float valve needle. Internally, however, the float is positioned in the above-mentioned float bowl and is connected via a metal arm to the fuel valve needle. There are also three circuits: in other words the idle, off-idle and starting (also called the Starter) circuits, separated from the others and to be used only for start-ups.

The following are the main components of the float carburettor:

1. Starting device; 2. Air intake; 3. Diffuser; 4. Starting float valve needle; 5. Float Bowl; 6. Atomizer; 7. Fuel valve; 8. Conical needle; 9. Fuel valve; 10. Float bowl air intake; 11. Fuel connection; 12. Idle mixture adjustment screw; 13. Fuel valve adjustment screw; 14. Float; 15. Idle emulsifier; 16. Idle float valve needle; 17. Off-idle float valve needle.

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[2] By turning the choke lever, a channel opens that brings air and more fuel into the engine, bypassing the fuel valve. This operation is necessary for the engine to run cold. The function of this circuit is to allow the passage of the air-fuel mixture even with the fuel valve completely closed. This allows the engine to receive a quantity of mixture sufficient to start-up and to ensure its operation when it is idle. This secondary circuit activated by the choke lever has its own duct, a float valve needle and a flow control element.
[3] The vacuum generated by the first rotations of the crankshaft is sufficient to suck in a “significant” amount of fuel, essential for start-ups and keeping the engine idle. In this phase the carburetion will be extremely rich.

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