MOTORCYCLE SPORT

The main Carburetor functions are: • To form a proper homogeneous inflammable mixture of fuel and air • To supply the engine with varying amounts of this mixture The fuel-air mixture is formed through vapourising and by uniformly spraying fuel into the airstream or at least by atomising it into very small droplets. Atomization takes place in this way: liquid fuel from the atomiser nozzle meets the flow of air which carries it, broken into very fine droplets, to the combustion chamber. We have spoken of a “proper” mixture because the mixture strength, defined as the amount of air in weight mixed with a fuel unit of weight, must have a precise value, ie it must be within the limits of inflammability so that the mixture can be easily ignited by the spark in the  combustion chamber. lnflammmability limits for commercial petrol are: 7: 1 (rich limit ie. 7 kgs of air and 1 kg of petrol) , down to 20: 1 (lean limit ie. 20 kgs of air and 1 kg of petrol) .

To obtain optimum combustion between these inflammability limits, a value very close to the so- called stoiciometric value is needed ie. about 14.5 – 15.0 kgs of air to 1 kg of petrol. A stoiciometric mixture ratio is one which ensures complete combustion of fuel with only the formation of water and carbon dioxide. The stoiciometric mixture ratio depends on the kind of fuel used, so if the fuel is changed, this fuel- air ratio will also change (see SECTION 5.1 ) .

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