43 FARM MOTORS 



and rust will prevent contact. Any of the above sub- 

 stances between the ignitor points will prevent a spark. 

 The point of ignition varies with the speed of the engine. 

 On a slow-speed engine, one of about 225 revolutions a 

 minute, ignition should take place when the crank is 10 

 or 15 before center. This angle increases as the speed 

 of the engine increases until in an engine running about 

 700 revolutions a minute the angle is from 35 to 40. 



To locate ignition troubles is merely a matter of dis- 

 connecting certain parts of the circuit and locating the 

 trouble by elimination. 



574. Carburetion. If for any reason the carburetor re- 

 fuses to give a proper mixture, the engine will refuse to 

 run. In this case it is necessary for the operator to 

 assure himself that everything else is correct, then clean 

 out the cylinder by turning the engine over several times 

 and beginning as if he were starting the engine for the 

 first time. A too rich mixture is detected by black smoke 

 appearing at the exhaust. A too poor mixture is deter- 

 mined by a snapping sound from the exhaust, indicating 

 that the mixture is slow-burning and is still burning when 

 the exhaust valve opens. The gas engineer determines 

 whether or not his engine is running properly largely 

 from the sound of the exhaust. 



575. Action of valves. The valves now used on the 

 gasoline engine are all of the poppet type and give a quick 

 opening. An engine will not run if the valves are 

 not properly timed. The inlet valve is operated by 

 suction; however, a little greater efficiency is obtained by 

 having this valve open mechanically, as there is less op- 

 portunity for the charge to be throttled during admission. 

 The exhaust valve is always opened mechanically, and 

 should open about 45 before the beginning of the ex- 

 haust stroke, closing at the end of the stroke. 



