Hello I'm new to this forum and hope it brings me alot of help. Anyhow I have a 89 lt250r, bored .60 over, just rebuilt the top end. Every once in a while it just revs the **** out of it self, even if I'm not on the gas. I noticed once that I kicked the fuel off, but other than that, I don't see why it does it. I know its got gas, and I know that when something is running out of gas it starts to rev up but this thing goes wide open. The only way to kill it is 5th gear and brakes, turning of the key and kill switch, and it will keep running when this is happening. I wonder if the adjustable exaust valve has anything to do with it I don't know and really need some help. The last time I went ridding I let a friend on it and it did this around a hairpin turn and he broke his collar bone, I really don't want to break a bone if it can be prevented. Please help, thanks in advance

 

It's called pre-ignition and it's is caused from a lean mixture, too low of octane fuel or poor fuel, carbon build up creating hot spots in the combustion chamber and or incorrect spark plug heat range . The lower the fuel level is in the float bowl the leaner the engine will run, the leaner it runs the hotter it gets and the more easily it can pre-ignite. Heck I have seen 250R and Blasters pre-ignite so bad you can shut it off take the ket out, pull the plug wire off and it will still screem like it is wide open (it's not but it sounds like it)

You can try running a higher octane fuel like a 93-94, a colder plug, a cleaner burning oil (to reduce carbon built up) and make sure you don't run the fuel bowl dry
Do a search for pre-ignition and it' will give you more detail

 

http://www.atlanticjetsports.com/_techtalk/00000007.htm

 

Could it be a leak in a gasket this would make it lean and could make it rev on its on.Maybe do a leak down test and see.

 

due what hardly and sob said,make sure you fix it,it could get real ugly if you keep riding like that

 

my friends blaster had this problem with his blaster when he had it bored over also. it blew up about a month later then he had it bored again but the problem was still there. i can recall on one occasion when we were riding and it happened, he stopped pulled the clutch in, pulled his tether out for his kill switch and it was still screaming. he blipped the throttle a couple times then the engine finally quit. we went home drained the gas tank, and took apart the carb and cleaned it, adjusted all the throttle and clutch cables and it fixed itself.