cause of burnt piston??

[NORTHMANSX]

hey guys, just changing out a piston and not sure why it burnt, it is the pto piston which is burned on the exhaust side, down to the top ring. the rest of the piston is fine, no scuff marks anywhere and the cylinder is alright too. jetting is one size down from stock and original thick head gasket. any ideas or suggestions, thanks

 

[2ooosrx]

are you sure what you are looking at isnt "blow by"

 

[NORTHMANSX]

i am sure, the piston is missing from the top down to the ring!

 

[2ooosrx]

I am not sure then, I run a jet leaner all across the board and it seems to be fine,I had my motor open about 2 days ago to replace the rings.

 

[yamaholic22]

possibly a partially clogged main jet, dirty needle jet or too lean needle setting (if you changed it from stock), or a pto bearing seal leak. This can be checked by running the engine and spraying some either behind the primary clutch. If the engine's rpm's rise, the seal is leaking and needs to be replaced.

 

[Brad]

most likely the crank seal

 

[98srx6]

got a picture of the piston?

 

[NORTHMANSX]

i could post a pic of the piston, but i would have to figure out how to, haven't posted pics yet. but i will try to get one on tomorrow.

if it were a leaky seal wouldn't it seize up in the bore from leaning out rather than meltin the exhaust side edge? i could check for a bad seal easy enough but never considered it as the problem.

what causes detonation and what will it do the the piston? somebody suggested it may be that..

 

[DoktorC]

Lean burns the exhaust side off the piston. EGT gets so hot it melts the piston. Mine just did the same thing..bad fuel (99% sure).

 

[NORTHMANSX]

so your saying crappy fuel may have caused it?

 

[DoktorC]

Its another possibility..but likely its a crank seal..how many kms on the engine?

 

[NORTHMANSX]

i's got 14000 km, replaced crank bearings at 7500km.