[pete]

Had the same issue with the boys pit bike...
totaly a design f_ _ _ up... the swing arm pivot is
to low... on a real motor bike the chain gets tighter
as the rear wheell moves up but on these Chinese
things it gets loser.. So you have to set the chain tight
I put nylon disc each side of the rear sprocket to stop
the chain de-railing.. made them 30mm bigger dia than
the sprocket...
But the ultimate cure was to buy the boy a Honda CRF150...

Ps... the chain took out 2 ignition units & one side case before we sorted it...