KX 250 locking up, revving like crazy

[carlbielke]

I just got my '99 KX 250 back from the mechanic, after a complete engine overhaul.
After warming it up and one slow lap on my track, it locked up under full acceleration, screaming like crazy.

I held the clutch in and stabbed the stop button but that didn't work, and I worked the accelerator to see if I could get it unstuck but it took almost a minute to stop.
After that I started it and it worked ok. I was scared enough to stop riding and decided to park it for the day.
The next day I found that the carb cable was routed wrongly so I corrected that thinking that I fixed the problem.
I took the bike out the next afternoon, same procedure, this time I made it around one and a half lap before the same thing happened again. This time I closed the fuel petcock but it still took over a minute to stop, the water began to boil over and poured out of the radiator cap. Finally it stopped, and after letting it cool down and checking the radiator fluid (it only lost a small amount, 100 cc, which is about a tenth of its capacity) I kicked it over again, and it started on the third kick.
The thing is that now it sounds kind of tinny, and I could swear it feel like it's lost power.

The next day I tore the carb apart and found some grey oil and water on the slide, it seems the rubber cover between the top of the carb and the cable worked itself loose. I cleaned it out and yesterday I rode twenty laps and it didn't lock up so I guess the slide stuck.

What beats me is this:
What could make the bike lose power when revved like that?
It sounds tinny now, and there's a whining noise that goes with a lot of vibration?

What could have broken?
I'm stumped and if anyone could give me some advice what to look for when I open the engine again next week, I'd really appreciate that,
Best regards,
Carl

 

[MrLuckey]

My guess on the loss of power and tinny sound is that when it starved for fuel (and OIL) it toasted the rings. Who knows what else let go when it was revving at 12,000 rpm and lost fuel and lubrication. I'd suspect that whatever it was, it is the source of your vibration.

 

[chevy383cid]

That's true, I've seen guys turn there fuel off and let there bike run until its almost out of gas and then rev it a few times til it dies. just think at what that does each time to your piston,rings,and cylinder???I've had throttles stick before at wide open and just held the brakes and put it in gear and let the clutch out.

 

[carlbielke]

Yeah, I suspect rings too. Thanks, MrLuckey and chevy383cid, what you say makes a lot of sense.

I thought about letting the clutch out but frankly was afraid to damage the gears and thought it would be cheaper this way, hope I'm right.

Do you think I can ride tomorrow or should I just wait?
I have another set of rings but the cylinderwall Nikasil coating is notoriously thin on Kawasakis it seems.

Another thing I checked today was the powervalve, it sure seems to open early, and it takes a while to close too, I'm mystified as to why that would be but we're taking it apart Monday night, just to be sure.

 

[rickyd]

If it revs freely (to me sounds like an air leak of some sort) cover the exhaust outlet, the cyclinder will fill up w/used exhaust and die..

 

[carlbielke]

Ok, thanks for the tip, it's hard to think clear headed when you bike is screaming at 12.000 RPM,
I'll try that one if it happens again, I'm hoping NOT.

I also thought about an air leak, but in that case I should be able to repeat it, and I haven't.
Since I cleaned out the carb slide it's been running fine, and since the rubber bootie on top of the carb was off, I'm suspecting that a dirty carburator was the culprit.

 

[snb73]

Sounds like you had a lean condition, causing a hot spot in the cylinder. This will cause the fuel to ignite, even if you press the kill switch. If it happens again, try pulling the choke. This will cause a rich condition, cooling the "hot spot", stopping the auto-ignition.

Sometimes this can happen if you forget to turn the fuel on. It happened to me, I tried to remove the spark plug. Dumb move on my part, I got the sh#t shocked out of myself. I wish I would have known about pulling the choke.

Sounds like you need to chase down an air leak. This seems to be cqausing the lean condition.

Good luck, Steve.

 

[carlbielke]

Thanks Steve, I'll definitely try the choke next time it happens.
I got my bike back today, the rings were gone and the piston was scratched so after more or less seven minutes riding time I had to change them. The cylinder has some scratches so I'll replace it eventually but for now I'll leave it like that. My mechanic told me to break it in softly, I'll do that now and write a summary of this incident in a few days time, so others can learn from my mistake.

 

[carlbielke]

Update:
I've now been able to ride the bike a lot, and it works just fine. It seems to me like the carb stuck. I changed the throttle cable (just the cable, not the housing) because it was a bit frayed near the throttle and I noticed something unusual. The cable has a quick adjust nut near the carb and it had worked itself loose. Maybe that caused the cable to stick, and with a dirty slide the spring wasn't strong enough to return. It's just a guess though; from now on I'll be a lot more anal about carb cleaning and maintenance. Thanks to all who answered my question.