why is my engine blown

G

grachal

New member
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?
 
grachal said:
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?
Click to expand...


Out of interest , did you check the coolant impeller?
 
I am sure you know if the spark plug is always wet you have a problem. You are either getting way too much fuel or you are not getting spark or enough spark with the plug in the engine. Maybe you are getting a poor spark. Try to shorten the spark plug gap to something very small like 0.3mm.

Silly question, did you put a rag stuck in the airbox when you had it apart?

Did you replace the crank seals when you rebuilt the engine. The old seals go bad and give you a bad air leak which will cause you to go lean.
 
Thanks guys!

@Biff: The waterpump impeller was fine. I replaced the impeller, the drive gear and all the seals last year, and they are still good.

@Zman: Crank seals, of course are new, genuine viton ones. No rag in the airbox, so it should get sufficient air. Spark outside the engine is very nice and bright, inside might be a different story tho. I will try reducing the gap. I also will check the carburetor, it sat for a few months, maybe the float doesn't close. But it doens't leak out the vents so I didn't bother looking at the carb
 
Pull the crank case drain bolt and make sure you do not have any coolant in the crank case.

I had a similar experience after a rebuild and had a small leak at the coolant passage between the case halves.
 
If you had not said that you checked the timing I would suspect that the woodruff key sheared on the ignition side of the crank when your motor locked up. You might want to check your timing again.
 
@Jacobs 'Berg: Ok, so today I removed the crankcase drain bolt, but no coolant came out, just fuel.
I also removed the carb a few times, but did not find anything. The jets are clear, no debris or oil residue. The float valve works fine.
I also tried cranking it without the drain bolt, and it sputters out loads of fuel.
@gasser: Thank you. When I rebuilt the engine I of course had to remove the flywheel, and the key was fine. But maybe something went wrong assembling it. I will remove the flywheel again, and check if the key is there.
 
If a lot of fuel is coming out of the crankcase (not trans) bolt then the float in the carb isn't working properly.
 
grachal said:
@Jacobs 'Berg: Ok, so today I removed the crankcase drain bolt, but no coolant came out, just fuel.
I also removed the carb a few times, but did not find anything. The jets are clear, no debris or oil residue. The float valve works fine.
I also tried cranking it without the drain bolt, and it sputters out loads of fuel.
@gasser: Thank you. When I rebuilt the engine I of course had to remove the flywheel, and the key was fine. But maybe something went wrong assembling it. I will remove the flywheel again, and check if the key is there.
Click to expand...

Don't turn over the motor with the crank case drain bolt out, you will be pulling unfiltered air right into the crank case!

If you are getting proper air and fuel mixture, you are not getting a good spark, or do not have good compression. Or, as stated above the timing is off.
 
Thank you so much guys, finally she is up and running again! The carburetor was the issue. Even tho I tested the float by blowing into the fuel line and actuating it, it seemed to get stuck once there was fuel in the carb. Had it all apart again, and put it in a ultrasonic cleaner.
She pulls stronger than ever before and the idle is smooth like it was an injection system. Only thing that concerns me, is the quite noticeable powervalve rattle. But I will get it sorted out.
gassn.jpg
 
Short update:
just after 53 kilometers I managed to seize her again. The motor started to develop a high pitch squealing noise, but ran great so I tried to bring her home unter its own power. 200 meters further, engine shut off with no compression.

Now it starts back up, but doesnt sound all that great. I hope that the crank is fine. Maybe I get away with yet another new piston.
 
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