my brother just had a complete rebuild done on his '90 lt250r. we took our time breking it in, took it real easy on it for the first tank.
then encountered a few problems:
it would rev up unexpectedly w/o having done anything.(jetting is good)
on the third day of riding after we thought it had enough break-in, we opened it up... soon after, it sheared the counter balancer's key, causing it to vibrate terribly. we shut it down and took it home.

has anyone had a similar problem? and what caused it?
i recall someone mentioning a bent crank being a possible culprit once, but am not sure... if so, how do we check it?

 

The engine revving without any throttle input is a classic example of an air leak. you need to check everywhere for air getting into the engine without going through the carb. With the engine running, spray brake cleaner at all gasket areas, around the carb boot junctions, and the reed cage. You will have found the leak when the engine revs up a little more. Caution! Break cleaner will 'wash' the lubricating gas/oil mixture off of your bearings and cylinder walls, so once you find a leak, mark it and move on to find any others. Don't keep checking the same leak.
As to your other problem, I cannot help you with that. Possible the key or keyway was worn?
Good luck.

 

As for the first problem, either you have an air leak somewhere (probably) or you have the throttle cable bound up somewhere (when you turn the bars, does it rev up?). Second problem was the keyways in the crankshaft and counterbalancer wasn't inspected. Hopefully you can salvage both and fix it with just a new key. It's likely someone in a prior rebuild had the counterbalancer out of time, and the vibration weakend the key and egged it's slots out.

Greg Coats
'87 LT250R
'00 Rancher ES 2X4

 

My 230 revs up lots of times.I just wiggle the throttle cable around where it attaches to the carb.And Sometimes reposition the thumb throttle.This usually helps.

 

we haven't had time to do much work on it, but we did some at the dunes. the throttle cabe is not the problem, we lubed it there & it worked fine.

it does sound like an air leak. but i'm frustrated about the counterbalancer still... my fear is that this is going to get real expensive to fix properly.

 

from the way I heard it rev up when we were at oregon it seemed to me that the slide was sticking from sand or something . When i heard it it reved to like wide open so that is no air leak but I do have a pressure gauge and a vacuum gauge to test it if you want. As far as the counter balancer goes I would realine the balancer gear and the crank and put a new key in it again and see what it does.Because it seems like they were not properly alined when the motor was put back together and that what caused it to sheer. You can talk to urbie to get my number if you have any questions