KX85 Help...I'm confused

[Nick's #1 Fan]

I am so confused....a friend gave us a kx85 to do some flat track racing and we are having some serious problems with it. When my son went out for pracitce it ran fine for about 7 laps(indoor short track). Sat in line waited to go again made it 4 laps and the bike locked up. He was going down the back stretch and it locked up the rear tire and turned him sideways. I thought for sure he siezed it. I pushed it off the track and went to kick it over and it fired right up. He went again and made it 3 laps and it did the same thing. I thought that it may have been vapor locking so I checked all his fuel lines and gas cap. Everything looked and operated fine. I cleaned the carb and messed with the jetting with no improvements.

I thought that it may not be getting enough fuel but not getting enough fuel wouldn't make it die and lock everything up. My son has siezed a motor before and he said that it felt like that.

I can't figure out why it would lock up and then kick over. I was wondering if I was having a cooling problem and the cylinder was heating up enough to stop the piston from completing the firing sequence, which in turn stopped the bike dead in its tracks.
I am getting ready to tear down the motor but I was wondering if anybody has any idea what this could be. This bike has been sitting for awhile so I don't know much about it right now but It appears that I am going to have to get very familiar with it.

Thanks

 

[ignition retard]

If that happened to me I'd take off the expansion chamber and have a look up into the exaust port to see what the piston and cylinder look like. Perhaps check the float bowl for any possible debris that could block the jets and then drop back down. Drain the trans oil and look for metal pieces (who knows maybe the tranny is locking)

 

[Nick's #1 Fan]

Never thought about the tranny....That would make sense but There were a couple of time while he was waiting to practice it would just die....

I will check the oil just to make sure there are no other issues.

 

[SteveinSpringHill]

95% of KX85 piston problems occure at the exaust port "bridge area" additionally it is the greatest area of heat. Therefore a peek in that area is a very good idea.

Only other failure I've had or seen is from sand getting aorund air filter seal and scuffing piston and ring on intake side.

Tranny could be a couple things shifter spring or shifter fork(s) bent.

 

[whenfoxforks-ruled]

Send the cylinder to Forward motion. Have Eric get it replated and set up for flat tracking. It is seizing, catastrophic failure is next. Kaws' have the reputation for having the thinnest plating, and US Chrome the thickest!

 

[Jasle]

Send the cylinder to Forward motion. Have Eric get it replated and set up for flat tracking. It is seizing, catastrophic failure is next. Kaws' have the reputation for having the thinnest plating, and US Chrome the thickest!

this guy is right on track....siezing then kicks over when cool. the bridge gets hottest and cools quickest. check your jetting also. good mx jetting is not likely to be good flat track jetting!!! been there!