Piston failure?

[Rider]

Had a piston overheat and melt around the top ring close to the exhaust port on my 74 TM250.The symptons leading up the the failure were engine pinging and revs running on with the throttle shut in.I shut the engine down before It seized. The reason for this post is what I found was the top of the carby was unscrewed and only on by several threads? Could this allow enough excess air to to lean out the mixture to the point of overheating? I can eliminate a lack of oil in the fuel mix as presence of oil on big & small ends indicate good oil ratio. The piston is shagged but the bore honed up O.K.Also the top thread of the mikuni has a slot cut in one side approx 6mm wide extending half way down the thread which may have been exposed.
Any comments would be appreciated. Regards Terry..

[cyclegod]

That loose carb top will most definately be the cause, it's the first thing my club scrutineer looks at (loose carb tops that is).

[Rider]

Thanks cyclegod, I felt it was the cause but looking at the slide and needle assembly its hard to imagine alot of air getting through.

[Marc.com]

doesn't take a lot of air, sorry for that one but bet it was a rocket ship before it seized, at least the bore sounds OK.

[dkupf]

I had the same thing happen to me years ago at a Nindi Gully enduro meet on a Cr. The loose carb top was the only thing we could find wrong that may have been the cause of the trouble  Note don't change jets in your carb pissed at 10 o'clock at night    Darcy.