Why is your 1998 Yamaha YZ125 spark plug getting wet and it will not stay running?

Maybe it's running too rich.check the gear oil to see if it's clear or discolour.if it's discolour the seals in the gear section of the engine is bad and the gear oil will enter the piston chamber and it will foul the spark plug. this will kill the engine when you come to a idle or you're not using the correct spark plug