Carburators

The part that kicks off the magic of combustion.

Functions

The most direct meaning of "carbureting" is "the mix of a flammable substance with air". This is exactly the function of the carburetor, it performs the mixture of air and fuel before it is poured into the cylinder so that the internal combustion of the engine occurs.

Operation

The fuel inlet channel leads directly to the carburetor bowl (Figure 1). There it is stopped by a needle, controlled by the carburetor float (pin needle). While the bowl is empty and therefore not being fed by the fuel tank, the float tilts downwards and its needle releases the fuel passage. If the bowl is full, the float rises and the needle closes the fuel passage.

Figure 1. Lower view of the carburetor (inside the bowl). 1: slow jet; 2: pilot screw; 3: mixer tube and main jet; 4: fuel inlet channel; 5: seat.

When the throttle cable moves the pulley located at the top of the carburetor (Figure 2), the slide is pulled upwards and this creates a vacuum inside the venturi. The vacuum generated causes the fuel stored in the bowl to be sprayed into the engine cylinder. The air inlet, located at the rear of the carburetor, is generally open so that the vacuum responsible for spraying fuel into the cylinder is generated. However, it can be closed by a valve known as a choke valve. This favors less air entry and greater fuel entry.

Figure 2. Components of an XR 200 carburetor. 1: bowl; 2: lockwasher; 3: slide; 4: slide needle; 5: Throttle cable pulley; 6: pilot screw; 7: slow jet; 8: mixer tube and main jet; 9: connecting arm; 10: venturi; 11: collector; 12: air inlet and choke.

Carburetor adjustment is basically done through the jets and pilot screw (Figure 1). The slow jet, as the name implies, controls engine power at low rpm (revolutions per minute). It is found inside the bowl and is regulated by the pilot screw, which is located outside the bowl and can be tightened (less fuel is added to the mixture, less rich) or loosened (more fuel is added to the mixture, richer). The same goes for the main jet, which is located next to the mixer tube and controls the power supply to the engine at high rpm. The engine feed at medium rpm is controlled by the lock that is located in the "head" of the slide needle (Figure 3). The lower the lock is (inferior cracks), the higher the needle will be and the more fuel will be released for combustion. This happens because the needle has a conical shape (the upper region is wider than the lower one) and the higher the needle is, the smaller the obstruction of the passage of fuel contained in the bowl.

Figure 3. XR 200 carburettor slide needle.

At the garage

We found an XR 200 carburettor in almost perfect condition in the shop. See their assembly in the Mechaniclapse video.

References

Zimmerman, M. (2016): The essential guide to motorcycle maintenance: Tips & techniques to keep your motorcycle in top condition. Motorbooks, Minneapolis, EUA. [Link]