852 AGRICULTURAL ENGINEERING 



amount of power and then be slightly closed. The adjust- 

 ment which gives the least number of explosions does not give 

 the most economical setting of the needle valve, and that is 

 why the valve should be closed slightly. 



Testing the Mixture. The most perplexing trouble 

 comes when it is impossible to get a single explosion. In this 

 case certain tests must be made to determine whether the 

 cylinder is flooded with fuel or whether there is not enough 

 gasoline vapor present to make an explosive mixture. Of 

 course, tests should be made to determine that the ignition 

 system is perfect and that an explosive mixture would be 

 ignited if there should be one in the engine cylinder. 



One plan to follow is to shut off the fuel supply and clear 

 the cylinder thoroughly of all gasoline by turning the engine 

 over several times. This being done, an entirely new attempt 

 to start the engine will usually meet with success. 



A test may be made of the nature of the mixture in the 

 cylinder by holding a lighted match to the relief cock as the 

 engine is turned over. A rich mixture will burn as it comes 

 in contact with the air; an inflammable mixture will snap back 

 into the cylinder; and a mixture which is too lean will not 

 burn at all. 



The Compression. It is necessary that a gasoline engine 

 compress the mixture of gasoline vapor and air before ignition 

 or the full power of the fuel will not be obtained. Failure 

 to secure compression is usually due to leaks, either past the 

 piston or through the valves. 



Leaks. Piston rings are provided to make a gas-tight fit 

 between the piston and the cylinder. Sometimes these rings 

 become stuck in their grooves by charred oil and do not 

 spring out against the cylinder walls as they should. When 

 this happens, which usually is due to the use of poor lubricat- 

 ing oil, the rings should be thoroughly cleaned. Where the 



