[Dubhicks]

Lost the #4 piston on my 98 Yamaha V4 115. Any ideas? The original powerhead was replaced a year and a half ago for the same cylinder failing.

 

[Dubhicks]

Picture of the crown of the piston. Pink paint denotes the top of the piston with exhaust on the right side, and intake on the left.

 

[Dubhicks]

Side shot of the intake side of the piston.

 

[Dubhicks]

This is a shot of the bore. Note that there are no seizer marks on the cylinder walls. Just a few gouges from what I assume we're the ring biting.

 

[Dubhicks]

This is a fuel sample that I took fresh from the water separator. I did not see any separation in the sample, and very little debris in the bottom.

 

[Dubhicks]

I reomved the carbs and disassembled them. Found that the jets were clean and clear and the floats all worked the same. No difference noted from one carb to the next in respect to air fuel flow. Oil injection appeared to be working fine. Never got an overheat warning or lost water pressure at any time. No air leaks were found around the carbs or intake manifold. No idea what the timing was prior to the failure, but thing ran like a scallded dog until it failed. The quality of the fuel has been called into question, but with no other damage on any other pistons, that's a hard pill for me to swallow (even though plausible). Spark plugs are the factory NGK recommended and were changed during the spring service.

 

[DuckNut]

My guess would be a small water leak and water vaporized into steam.

 

[Dubhicks]

Was unable to find any water leak. A few folks have said it looks to have run lean. Thank you.

 

[Brett]

http://www.smellofdeath.com/lloydy/piston_diag_guide.htm

big pdf file: http://www.boosttown.com/engine/piston_damage.pdf

 

[noeettica]

VERY NICE !!!

http://www.smellofdeath.com/lloydy/piston_diag_guide.htm

big pdf file: http://www.boosttown.com/engine/piston_damage.pdf

 

[Dubhicks]

Nice articles. Good read. Thanks.

 

[Dillusion]

I had a two stroke merc go out on me from running too much seafoam/cleaner through it causing it to go lean...maybe related

 

[priscillatgorilla]

too much prop can and will cause a detonation,like that

too much pitch,it overloads the motor - exhaust temp rises and "boom"...


seafoam - it's like hand sanitizer,the main ingredient is "paranoia"...

a carb with debris in it - the debris needs to be removed - which means disassembly and cleaning

 

[swampfox]

what did the plug look like? You got a pic of it? If it was run lean it would be vey white/ashy. That damage kinda looks like the bits of the ring smashing around. There were two rings n that piston right?