Bike dies, whats the problem

[Dirt_Rider19]

i was riding my bike the other day all day at the local track and we decided to for a ride in the trails ridin wide open to 4th gear most of the time and on the way back bike like bogged and just died while i was riding. i put a new plug in and nothing. did a compression test and was only like 30 psi. its a 2004 cr125r. i took the cylinder head off and not a scratch in it but the piston had some scrapes on it. just replaced piston and ring and cylinder maybe 10 hours ago.

 

[rmc_olderthandirt]

Wide open for a long time, bike bogs and dies, wont start, very low compression.

That is what I would call a seize, even if it didn't actually seize up. For what ever reason the ring isn't sealing anymore.

How did the piston get "scrapes" if the cylinder is perfect?

If the cylinder is actually perfect then I would get a new piston, ring and gaskets and put it back together. Make sure you check the end gap on the ring as that could have been too tight and caused this failure.

Rod

 

[whenfoxforks-ruled]

The rings froze onto the piston. The piston has left aluminum flashing on the cylinder. The cylinder needs to be properly cleaned and inspected. The bottom end needs to be inspected, very carefully. The aluminum flashing has been washing the bottom end also. If you have the money, I would rebuild the motor. Vintage Bob