While out at the weekend my WRF came up with a new idea on how to spit me off. On steep down hills, on or off-road it will have good engine braking, with a bit of a backfire, for 3 sec's, then the motor starts to race (up to about half throttle) forcing me to pull the clutch. If left alone it'll race for about another 5 sec's then calm down again. The carb is clean and clear of water, the air filter's good and it'll tick over quite happily off choke (once warm). Has anybody come across this one before, or has the little devil invented something completely new. Once again, any ideas, even if it's just where to hit it with a hammer, will be appreciated.