!!! Important, Dirtbike Owners !!!

Check out the ouchy, from my 97 Kawasaki KX-80. Two stroke owner's, don't ride at constant throttle, work it on and off. I learned the hard way.
Here's a link on that piston seizure (http://www.dansmc.com/seizure.htm)

well, at least you don't have to worry about poppet valves. ;)

What fuel/oil ratio are you using(or is it an injector system)? Piston seizure is fairly common in older 2-stroke designs, but usually it's is only partial seizure and once the engine cools down everything will still be o.k. and free itself up (not the case here!)
2-strokers can handle constant throttle, a friend of mine who owns a Subaru Sambar van only ever drives with the pedal all the way to the floor (60mph top speed) and he's been doing so for the past 20 years! - without ever siezing the engine. Most commonly the engine will seize on hot days, usually I've found going up long stretches of hill, you'll feel the engine lose power, almost as if somebody is applying the brakes, this is your signal to let off the gas - but never turn the engine off! - it can sieze solid if you do! Letting off the gas has the effect of cooling the engine, but keeping it moving. Also, don't add extra oil to the gas if you think this will solve the problem, it'll lean out the fuel mixture and also cause seizure.
And well, at least you don't have to worry about poppet valves ;)

Hmmmm, I read your webpage on the subject. I'll have to disagree. It is only possible to ride a moped with the throttle all the way open all the time, you have to because they top out at 25-30mph. I have ridden for as long as two hours on my moped with the throttle in the same spot, and never once seized it. It is perfectly happy though to stay in any rev range for any period of time though.
Also, loop scavenging was prefered over reverse flow and finned piston designs because it was believed that it offered the least mingling of the fresh charge with the exhaust gasses, so unless you have extensively modified port timing, the amount of cool gasses actually traveling over the piston and out the exhaust port is very little (more so at idle, because of the bad scavenging characteristics of this port design at low engine speeds)

somthing either got in to your engine or you were running the wrong gas-oil ratio, or way way to lean. i ride my 94 ktm 250 in the red all the time and have yet to break it (then engine but every thing else i have!)

yah my moped can olny go 20(not for long) but it will do that all day

You may all be right about the link being wrong, but the link points out something that kind of debunks your counter points...An engine is designed according to it's use. A two stroke lawnmower or outboard engine is designed to run at a steady speed. A two stroke motocross engine is designed to be run full throttle, part throttle, full throttle part throttle. On and off, on and off. A moped, or van would be designed to operate under constant throttle...like the quote says, a motocross bike is not...I own a motocross bike. Oh and to answer your question about mixture, I run my bike on a mixture of 20:1.

O.K. then, please point out the differences between a "constant speed" engine and a motocross engine, that might lead to a motocross engine siezing when run at a constant speed. 20:1 sounds a bit too rich on the oil too, oil displaces gas, which in turn leans out the mixture, causing the engine to run hot, and possibly sieze - how long have you been running that mixture? I'd move up to a larger jet size, or take the fuel/oil ratio down a little if you have this problem again.

those piston rings also look like they've been eaten away over a bit of time, not suddenly.. unless they're lodged in the cylinder wall. A bit too much oil it seems to me too... but it could be just a freak thing.

Well scuffing does happen when a 2-stroker sucks in dust, I hope he wasn't running without an air filter (especially on a dirt bike).

I am not sure how it was maintained before I got it, and you guys probably know better than me...I'm not a 2 stroke expert or anything, but I do understang the principles. I am not sure what the difference between a "constant speed" 2 stroke and a motocross 2 stroke is, maybe there isn't one. I was told to run it at 20:1 by the guy who sold it to me, and maybe that's what the problem is...I just figured it sounded more like seizure as a result of running at constant speed (as I always keep my bike well maintained and have never had any problems with it before). Oh and about the piston rings, the reason they look so bad is the piston is aluminum, and melted. When the piston broke free (I was moving while it seized and the wheel broke it free) it smeared aluminum over the rings. And of course I use an air filter, and I keep it properly oiled too.

Sorry for digging up this old thread, but the answer to this perplexing mystery has finally been found. A slightly related topic came up on the vintage saab mailing list and I figured out why some 2-stroke engines will sieze when run at a consistent speed.
Because the lubicating oil is mixed in with the gas on most 2-stroke engines, this means the amount of lubrication the engine recieves is controlled by the throttle - this is why you never close the throttle when going down hill in a 2-stroker, because the engine will move at a rate higher than can be lubricated properly with the throttle in the idle position.
However the same thing can happen when you're running the engine at a consistent speed, especially when the throttle is only partially open, the oil coming into the engine will not be enough to keep the engine lubricated, but when you vary the speed from high to low you get different ammounts of oil in the engine at various speeds.

So there you have it, get a good synthetic oil as this will help keep this from happening, and vary the speed to keep enough oil in the engine.
(the reason I've never had this happen to me, is that the Subaru uses an oil pump, so it is not reliant on oil from the carb to keep lubricated, and the moped is kept at full open throttle, which means maximum lubrication)

Thank you for finally clearing this up, I knew there had to be some truth behind the theory.