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yamaha 40 hp

T

tony cole

New member
hi all im new to this site and hopeing somone can help me rite here goes I have a 40 hp yam 1989 itsits loosing power I e not reving out I noticed the bottom plug was black and wet so I done a compresion test all cylinders are 135 psi its sperking very well so I thought petrol I cleaned carbs then noticed the top and middle cylinder it kills the engjne wen you turn the mix screw in but the bottom one dont make no differece at all turn it fully jn and nothjng happens at all plz help me
 
Reed valves ok on the bottom cylinder.---------" sparking very well " means nothing here.-----------Answer this , will spark jump a gap of 3/8" or more.
 
You need 4 things for that motor to run.--------------You have strong spark.----------You have compression.------You say carburetors are clean,-----Next is crankcase compression.----- and you say reeds are good so that leaves the lower crankshaft seal as suspect.
 
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