RM250 90 RM 250 Starting issue


Staff member
The damage is not indicative of detonation. Those stamped lines are cause by physical damage of a piece of piston bouncing around in the combustion chamber.
Detonation may have initially cause the piece to break off, but the rest of the damage is physical contact.
Yep it was definately chewing on something. And like some of the other guys suggested tear it all down and I do mean all of it. And rebuild the entire thing. Or sell it for what you can get for it. The down side to doing a complete tear down is that it will cost probably as much as the bike is worth or more do it. Dont get me wrong Im not saying your bike is cheap. Its just old and almost not worth doing it. Unless..... You really love that bike. Truthfully I will end up doing the same thing to my bike and its only 2 years newer than yours. But its only because I LOVE my bike.
That float level is key on this bike, 14mm is what you're shooting for for the 1990, measured from the brass tangs that the float touches, as close to WHERE the float touches, to where the main jet screws in, some find it easier to take the main jet out to get it outta the way but you don't have to, use a digital micrometer, on the tail end of it which is best for measure depth and stuff, works great, make sure spring on the needle isn't depressed, hold the carb at around a 45 angle otherwise the dry weight of the floats will depress that spring and mess up your measurement. Measure both sides since these tangs bend easy, you may have bent one at some point when you had carb apart for clean and jetting. Float height is the first step to carb tuning, and also causes a lot of these issues your having still.