Piston crown crumbling away...?

[mick1]

One lesson to be learnt: After a problem like this it should have been leak tested before strip down, would have stopped a lot of guessing and surmising. I leak tested my Avanti when I lost compression, a leak test before strip down showed a leak from a damaged casing, saved a lot of time and doubt.

Hindsight eh !!

[Fruitjuice]

I would agree that when you look closely at two stroke piston damage that it's not always clear what caused it, and in a chinese parliament that is a forum you'll get many different answers. Partly through hearsay, partly through personal experience. History is MASSIVELY important in tracking down a running problem. Your piston has overheated close to/ at TDC. Identified by where the plating / aluminium underneath the plating has become unstable due to heat. The damage on the crown of the piston is due to heat away from the edge of the piston, spreading across to where it can exit, the exhaust port. Either with a piston crown matching the bottom of the exhaust port or slightly above wouldn't cause, with an historically well running engine, this type of damage, as if it was there would be damage/ overheating below the exhaust port. Your picture would suggest that the exhaust port bottom edge is ABOVE the bottom edge of the transfers, therefore I cannot see that as the issue. There is no discernible ring damage, nor marks on the piston or cylinder to suggest any, let alone excessive piston slap/ rock. The piston also has not had time from the introduction of the excessive heat, to expand the piston to create more damage to it, i.e. there are no significant seizure evidence on the side of the piston below the damage. Historically (you tell us) it's well set up and has done some hard graft (track days). It isn't Compression ratio (dished crown, bigger build up of heat in the piston over the whole time the engine has ever run). It isn't jetting (would have happened sooner), it isn,t timing (ditto jetting), it isn't exhaust (would have happened on a motoway/ long fast run [back straight of track]), it isn't lubrication (piston has not seized). This is caused by HEAT. It's the two stroke/ air cooled enemy. What's left to create heat? Do you know your previous afr readings per throttle position? Since filling up, was the last 15 miles on the motorway, or the first 15 miles?. What was your throttle position at when it happened? The irregularity that caused this was instant, by the nature of the damage surrounding the failure. Instant air leak (carb falling off/ split rubber, possibly head gasket [but would expect seizure/ holed piston first]. A slightly weaker area of carburation range (throttle position) combined with ethanol rich fuel Will cause this. Especially if, historically it's a sound engine. If you were OFF full throttle when it happened, that's where my money is. Yes, I'm aware you are in Belguim. I have two examples of exactly the same damage of otherwise historically perfect engines.

Answers like this is why I love this forum!
After fueling up we did about 15 miles on B-roads and then throttelled it for the last 15 miles home sitting at close to full throttle. It was running perfect until I backed of the throttle on the offramp and limped home the last 1,5 miles.