I have A 1992 Rm 125, The engine was completly rebuilt and has about 20 hours on it. Now when I start it will idle but after about 3 minutes it just starts to rev to the limit?? I have cleaned the carby out and the throttle cable and slid seem good, new sparky etc. Any ideas??
My guess is that you have an air lock in the coolant line some were.....and it's over heating, what the spark plug gets to hot it will make the bike scream to the max and the only way to stop it is to stall it out.