The ABCs of Carburetion

UM-900 (1959)


Page 5 of 19

When the throttle is wide open, the carburetor is supplying all possible air. Since no further air is available, the only way to increase power is to make certain that all the incoming air is used for combustion.
In spite of the best efforts of designers, per­fect distribution is never reached; the average mixture supplied to an engine may be com­pletely correct, but some cylinders will run slightly rich and others slightly lean. So with no more air available through the carburetor, the lean cylinders are the only source of un-
burned air. The mixture supplied to the engine is richened until the lean cylinders become normal and all possible air is used for combus­tion. Of course, the already rich cylinders will be made slightly richer, but the excess fuel simply travels out with the exhaust gases.
Now it can be seen that for maximum power, extra fuel is added to use all available air and fuel economy becomes secondary.
For operation other than maximum power, it is desirable to supply an air-fuel mixture for maximum economy. Economy requires the re­verse of maximum power, in that enough air is added to insure the use of all available fuel, so that no fuel will leave the engine unburned.
The majority of driving conditions allow an economy mixture, but in some cases economy must take a back seat to power. So the auto­motive carburetor must be able to supply either mixture to the engine, according to power demands.



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