why is my engine blown
 

Hey,
yesterday my 1998 EC250 left me stranded with no compression. Pulled the topend off to find a slightly seized piston (rings stick to it on one half) with light score marks, cylinder looks reusable.
But the crank is absolutely done! it has a bluish heat haze in the conrod area and the rod bearing has like 5mm of up and down play, so I suspect all of the needles must have fallen out. The nylon counter weights melted away.

So my question now is, what might have caused this so quickly? Engine ran fine without any noise, I run 1:40 mix, with premium castrol 2t oil.

When the bike broke down, I checked the coolant at first, and it was weird, when I opened the radiator cap, the level was okay, but there was no pressure in the system. Was it overheating because of this?

I am sorry to hear that. It might have been lean? How is it jetted? Is the float level set correctly? Have the crank seals been replaced? What did the plug look like? Under what riding condition did the bike seize?

Thanks for your reply. The bike never "felt" lean, but who knows. Main jet is a 185, pilot jet a 38. I never messed with it, I think this is stock jetting. Float level I need to check.
According to the previous owner, engine was never touched, so the crank seals might be 20+ years old - like the rest of the internals. But the idle was always great.
Spark plug is light brown/grayish.

Riding condition might have been the killer here. The bike is converted to a supermoto and I rode it on the street for approx 4 hours now. It seized cruising at steady mid range rpms, but I assured to let it relax every few minutes and always pull the clutch in for a bit on longer straight sections.

that is most likely the cause, may have needed to jet it richer on the main. sounds like a heat sieze and you get heat when there isn't enough lube

When i used to race off-roads on a cr250, on the long flatout gravel road sections we used to pin it for 10 counts WFO, then hold the kill switch for a count or 2 (to pull in an open carb-load of unburnt fuel to keep everything well lubed) then flat out again - i never won, but also never had an engine seize

Ok so your piston doesn't show detonation yeah? That's the crucial question.

Big end failed, because maybe its 10 years old or more, debris from cage sends shrapnel up the transfers, hence seized ring which happens in a microsecond. When the roller bearing stops rolling they wear flats and fall to bits immediately but will get pretty hot in those last few seconds. Alternatively the plastic stuffers on crank can back a fastening out.


Competition bikes have set hour limits or miles for rebuilds. GG are so reliable I've never bothered recording it. But then I'm play riding. Road use means higher continuous revs. I'd consider your rebuild regime going forward.

Check out a TZ or Honda RS250 if you want to get scared about Competition miles to rebuild.

Okay, I'll rebuild the engine no matter what. Also I'll go up a few sizes on the main jet.
How do I check the piston for detonation? The top surface has a uniform brown color, with a normal amount of carbon buildup, also in a uniform pattern on it. So I guess no premature combustion? Also we have pretty good fuel in germany, and I always use 98 octane

And another question: Where do I get the nylon counterweights? I can't find them online

Google 2 stroke piston failure detonation and select images.

If it looks like any of those pictures I'd go up a couple of jet sizes. If it doesn't - I wouldn't as it isn't detonating.

Actually bikes can nip up on closed throttle and that could be pilot jet.

But it still sounds like your cause is simple bottom end bearing failure.

If it happened at part throttle, I would look at the next size larger pilot jet.

Hello guys,
so I finally managed to rebuild my engine, but sadly I can't get it to run. Spent the last couple hours kicking it, also trying it with an electric drill, it started up once for a few seconds, but thats it. It seems that my compression is very high, because kicking it is a nightmare compared to before. Can that be the reason why it doesn't start up?
I measured squish with solder and its 1.9 mm on the outer edge, so that seems ok, right?
Of yourse I checked fuel, spark and timing, everything ok. Sparkplug is always wet.
Should I put an additional .25mm base gasket and try again?

Out of interest , did you check the coolant impeller?