My friend just bought his sons brand new bikes a 01CR125 and a 01 CR80. His youngest son(11) forgot to put the air filter back in his CR80 and rode the bike for two days on a dusty track until the bike started to spit and sputter a high RPM's then we discovered that the air filter was missing. The bike still runs with good lowend response it hits in the midrange then it starts to spit and sputter. So far we've cleaned the airboot and the carb(main jet was pretty clogged,float bowl had some dirt in the bottom,airboot had a layer of slime on the bottom)then we took the pipe off and looked in the exhaust port the piston looked fine no scratches on it or the cylinder walls.Still no topend power though.Could the bearings in the crank be bad?Or should we replace the piston?I don't know what to do next. I need someone who has experienced this before.