Motorcycle Repair: over reving kawasaki kmx 200, kawasaki kmx 200, crankshaft seal

Question
Can you tell me why my 1989 kawaski kmx 200 revs straight up to 7000 revs on start up and stays there. I tried replacing the throttle cable and tried a different carburettor on the bike and nothing changed it did the exact same thing, revs straight up to 7000 revs and stays there. Could it be a reed valve problem or the kips valves. The problem occurred when I took the bike out for its first ride when I bought it, 10 miles into the ride the throttle wouldn't shut down. On inspection I noticed part of the throttle cable had melted against the exhaust so I tried the changes mentioned above but no joy.

Answer
Ronnie,Pull the air cleaner and inspect the slide the carb. If its up off the bottom of the carb it needs to be adjusted down. Since you have replaced the carb and cable then check the twist throttle on the handle bar it may not be in the proper position holding the cable tight and pulling up the slide. The melted sheath on the old cable may be because it was'nt routed right. Worst case scenereo is that a crankcase seal is damaged or dried out and the engine leans out and will rev up uncontrollably. Commonly its the left one under the flywheel and stator as it get no lubrication. Ive had some in the past that the crankshaft seal dried out and went to start it and about launched the bike off the lift before I could get it shut down. Check the throttle first then move on to checking everything.