49§ ENGINEERING ON THE FARM 



or overflow provision. The level of the gasoline is adjusted 

 so that it stands at the mouth of a small opening called 

 the needle valve or spray nozzle. An air chamber passes 

 directly by the orifice of the needle valve. When the air 

 is moved through this passage by the suctional effect of the 

 engine piston, there is produced around the needle valve a 

 slight reduction in pressure, which causes the gasoline to 

 flow out of the orifice, and when the suctional effect is suffi- 

 ciently strong, the oil is sprayed into a finely divided vapor 

 and carried into the combustion chamber. If the fuel used 

 is heavy, it is necessary to reduce the size of the air passage, 

 thereby producing a higher velocity of air past the spray or 

 metering tube. 



Adjustment of carburetor. An ideal carburetor should 

 require no adjustment after the engine has started and has 

 become thoroughly warmed. Adjustment is a matter of 

 experimentation. The desired end is to get the greatest power 

 possible from the engine. The possible adjustments involve 

 the fuel or the air or both. Too lean a mixture will be. evi- 

 denced by slow speed and lack of power and sometimes a 

 missing of explosions or backfiring through the carburetor. 

 Too rich a mixture will be indicated by weak explosions, 

 possible missings, and a black, sooty exhaust. Each particu- 

 lar carburetor has its own special procedure by which proper 

 adjustment may be most quickly accomplished. General 

 instructions only can be given : Open the needle valves until 

 the engine can be started. Let the engine warm up. Then 

 cut down the fuel until it begins to miss. Again open the 

 needle valve until it runs smoothly. With an auxiliary air 

 adjustment it may be possible to loosen the retention spring 

 and allow a larger proportion of air to enter. 



IGNITION 



Rapidity of burning. After a charge of fuel and air has 

 been drawn into the cylinder, it is necessary to have some 

 reliable means by which it may be fired. It is important 



