Why ? ?

[shepster]

Ski was running fine all weekend. It fouled a set of plugs so changed them and went out again all fine, bogged abit on the way in but figured it was due to lack of fuel in tank. Go to workshop today to do routine check and find this. Front cylinder 50psi rear 190psi as normal argh.
What would cause this as the rear cylinder looks rich if anything ?
Motor is 780 ported 61x running av gas, single 49 full spectrum.

Many thanks

James

 

[Matt_E]

Looks like you snagged a ring on the exhaust port, causing piston breakage.

I had the exact same thing happen to me a while back.
Come to think of it, you got engine carnage just about as often as me.

 

[shepster]

Tell me about it, i was only saying other day how good this motor had been, doh should of known.
Hopefully get away with a hone and just a piston as the bore looks to be good. Can hear the bits rattling in the waterbox.
So i guess the solution is to chamfer the ports more ?

 

[Matt_E]

I guess. The port shape you got there really isn't all that bad on rings, so it may have been the chamfer.

 

[shepster]

Well il see what i can do at work tomorrow, fingers crossed.
Would you say the rear cylinder looks ok, is the piston wash good or to rich ?

James

 

[Matt_E]

I'm not very good at this, but it looks okay to me, if not a bit rich.

 

[shepster]

Sweet, cheers Matt.
Off to bed now so will get things sorted and piston ordered tmz.

Many thanks

 

[SJBrit]

Well il see what i can do at work tomorrow, fingers crossed.
Would you say the rear cylinder looks ok, is the piston wash good or to rich ?

James

I'd say a little bit rich, but not too far off. I wouldn't change anything if it's running fine (apart from the fact that it's not running at all...!)

 

[shepster]

Hi thanks !

Now after the mist has settled and the bore has cleaned up nicely i have discovered what i believe to be the cause of it all. The porcelin of the plug has fallen out ! There is no damage to the plug other than its missing, Not beaten up by the debris etc.
Does this seem possible ?

Anyway pistons ordered so should be back on the water for the weekend :brap:

 

[Matt_E]

I am probably wrong, but I don't think that was the cause for the ring and piston breaking. The plug may have got damaged by a piece of ring.

 

[Big Kahuna]

That could also be foreign object damage from the Crank Bearings coming loose, that happened to me a few months ago, rear cage started to come apart, pisted looked just like that, in my case some of the material got between the piston and cylinder walls and cracked my sleeves. You might want to check your crank.

 

[shepster]

Hope not the crank is only a season old ! !
Will split cases just to be safe i think.
Could it be fuel as we have just got a new brand and there are a few people that have had simular problems on the exhaust side ?