8HP 1997 2cyl, 2 stroke Yam

[w2much]

Motor runs pretty good in a test tank on two cylinnders. Its has good spark but fails a drop down test. While running I can pull the top cylinder plug wire and it will run on the bottom cylinder. The spark is strong on the top cylinder. If I replace the top cylinder wire it resumes on two cylinders. When I run the motor then repeat the test and pull the bottom plug wire the motor stalls. It will not run on the top cylinder alone although it does on the bottom cylinder. I switched spark plugs and wires and still the same result. I would suspect the head gasket if there was one , this engine does not. I suspect water intrusion from the exhaust plate as my perp. Does this sound right ?

 

[racerone]

Does spark jump a gap of 3/8" on both leads , yes or no ?----Post results of a compression test.------Could be an issue with reed valves ( crankcase compression ) as well.

 

[w2much]

No, on both leads it does not . The bottom lead will produce spark at 3/8" with a strong yank only once on the first revolution. The upper needs to be at about 1/4 " to show spark, again on the first revolution. The compression is 102 top and 108 bottom.

 

[racerone]

Time to trouble shoot the ignition system.----I think there is an issue there.

 

[boscoe99]

Motor runs pretty good in a test tank on two cylinnders. Its has good spark but fails a drop down test. While running I can pull the top cylinder plug wire and it will run on the bottom cylinder. The spark is strong on the top cylinder. If I replace the top cylinder wire it resumes on two cylinders. When I run the motor then repeat the test and pull the bottom plug wire the motor stalls. It will not run on the top cylinder alone although it does on the bottom cylinder. I switched spark plugs and wires and still the same result. I would suspect the head gasket if there was one , this engine does not. I suspect water intrusion from the exhaust plate as my perp. Does this sound right ?

???

 

[racerone]

What he is saying , no head gasket on this motor !----Sort of built like many Mercury small motors.

 

[boscoe99]

If the motor runs pretty good on both cylinders why was a drop test performed?

 

[boscoe99]

What he is saying , no head gasket on this motor !----Sort of built like many Mercury small motors.

 

[racerone]

It is a head cover gasket.-----A true head gasket would have round holes.-----That head cover gasket has nothing to do with compression.

 

[boscoe99]

Interesting. I learned something new then.

How is the piston installed? From the crank shaft side?

 

[racerone]

Correct.----Small and even the inline 6 cylinder Mercury motors used this design since the early 1950's

 

[w2much]

So weak spark has been determined. I will start at the plug wires.