Oil? Seized? Piston rings?

[AlpinePossum]

So my blessed son elected to swap his bike for a larger (but older) KX250 2 stroke.

Took it out for a run today and after going over a jump, bad noises were heard and it ground to a halt. Restarting died promptly.

When I tried turning it over, the piston moves but is stiff.

Yes, it has right fuel/oil mix, spark plug shows fouling rather than too hot.

Crank case window shows oil present to right level.

Any guesses?

Thanks!

[ellipsis]

....bad noises...such as...?.....

[psycho22]

....bad noises...such as...?.....

Anything along these lines?

RRRIIING DING DING DI-KERPLUNK-Screeeeech-pop-fafrt-pop-chug-chug-splutter-splutter *die*

That about sums up my altogether rather bad experiences of two-strokes.

[tigertim20]

So my blessed son elected to swap his bike for a larger (but older) KX250 2 stroke.

Took it out for a run today and after going over a jump, bad noises were heard and it ground to a halt. Restarting died promptly.

When I tried turning it over, the piston moves but is stiff.

Yes, it has right fuel/oil mix, spark plug shows fouling rather than too hot.

Crank case window shows oil present to right level.

Any guesses?

Thanks!

id say your list of things to do before riding it again will consist of stripping it down, rehoning a tad oversize, fitting a new O.S. piston and rings, and probably having a looksie at the crankshaf bearings while youre there too.
but its a smoker, so not really that big of a deal.

[scott411]

it will most likley have a nikisil bore so no oversizing is possible with out a sleeve,

it sounds like it could be a bottom end failure, but has said above, it is a 2 stroke so not much to go wrong,

[Bender]

Chain too tight - trashed bearings in the gearbox? Tweeking the suspension to the max over the jump has caused catastrophe.

[F5 Dave]

It's a competition engine. In their day they were meticulously maintained & fed service parts at the appropriate schedule & were 100% reliable in racing conditions. The fact that it has gotten old & out of date doesn't mean it has got hardier, even if trail riding is softer. Stump up for the rebuild & get other parts like Powervalve inspected & you'll be sweet, just thank fuck it isn't a modern 4 banger. clean it well & Strip it down as much as you can rather than pushing a muddy bike to a shop & saying fix it.