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hi i am hoping some one will be able to point me in the right direction.

i rebuilt my 08 crf 250r. the bike is hard to start when cold but once hot is impossible. then i took it for a ride in some hills and it got hot when i stopped to talk to a mate the bike stalled on me. when i tried to kick it it spewed out wight smoke from the exhausts. i left it to cool after 10 min the radiator was cold but the engen still very hot. when the bike is running it sounds great only strange noise is when u really twist the throttle there is a crack i don't know how else to describe it. its not a back fire. it cracks then halls rear.

sounds like you did not put the head or cylinder gasket on correctly, but impossible to say for sure. The white some is likely coolant, which is why the radiators are cool - no hot water flowing through them.