Two Stroke Carburetor and Ignition System
There is nothing quite so simple, and at the same time complicated, as a carburetor. Your dictionary will tell you, correctly, that it is merely a device for mixing fuel (in this context, gasoline) with air, and although early examples employed several different means to this end it has long since been proven that the job is handled best by directing the air through a venturi in which a fuel-entry nozzle is incorporated. Pressure within the venturi is reduced below atmospheric in direct proportion to air velocity, which applies a suction to the fuel nozzle in the same proportion. Thus, fuel is drawn from the nozzle at a rate closely tied to air velocity, and the basic proportions of the combustible air/fuel mixture produced by the carburetor may be established by varying the cross-sectional area of the fuel entry as compared to that of the venturi. And, in point of fact, early carburetors were little more complicated than the device just described, with only a controlled-level fuel reservoir (the float chamber) and some form of throttle valve added. Modern carburetors are still built around the basic venturi, fuel nozzle, reservoir, and throttle, but various details have been added. The most important of these is the “correction-air” feature, which is needed to compensate for the venturi/fuel-nozzle’s great inherent defect: Uncompensated, the basic carburetor will deliver an air/fuel mixture in which even higher proportions of fuel appear as air velocity through the venturi increases. The reason for this is that the pressure-drop within the venturi is accompanied by a reduction in air density, and while flow from the fuel nozzle is in direct proportion with air velocity, the actual mass of air passing the nozzle does not remain in proportion. In consequence, mixture strength rises with increases in velocity unless measures are taken to prevent that from happening……….