The ABCs of Carburetion

UM-900 (1959)


Page 6 of 19

The extreme cold weather test of a carburetor is the starting of a cold engine. Cranking draws the normal amount of fuel into the engine, but due to low temperature much of it does not vapor­ize. Also, some fuel condenses on the cold manifold, so that the mixture reaching the spark plug may be too lean to start combustion. To counteract this leanness in a cold engine, extra rich mixtures are supplied to the engine
by "choking." Air flow is restricted so that more fuel is drawn into the engine; thus the air-fuel ratio is richened. As the engine warms up, vaporization and condensation conditions improve and the choke is gradually opened to prevent needless rich mixtures.
The problem during hot operation is the occurrence of vapor within the carburetor. The combina­tion of engine heat and high outside tempera­tures often makes the gasoline evaporate with extreme rapidity; sometimes it actually boils.
Good vaporization is fine for distribution, but the carburetor is designed to meter only liquid fuel, so vapor within the fuel passages disrupts the metering of the fuel. Vapor trouble will show up particularly at idle speeds, while
metering is being done through very small holes; vapor bubbles in the idle channels cause metering to be erratic and the engine runs rough or stalls.
Insulation and venting are the two general methods for fighting the vapor problem. In­sulation of the carburetor as much as possible from extreme engine heat helps prevent vapor formation; if vapor does form, it can be vented to the atmosphere in such a way that no liquid gasoline is carried along.



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