UK 04 WR450 Backfiring then motor races!!

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.:lol:

Double check the carb mate, i had a similar problem on my 1st lap of an mx track............... almost flipped it on the first jump as it felt like the throttle stuck open!

Sounds almost like a cracked slide plate in the carb. Although I've never heard of it happening in the later model WRs.

Could also be a leak in the rubber between the carb and the cylinder, it's running lean...

lean - check float level, check blocked pilot circuit or crud in bowl, were you low on gas?

Could also be an air leak between carb and intake.

Also - what were the temps? A bike jetted right for summer heat will run noticeably lean below 10 deg celcius.

lean - check float level, were you low on gas?

These two would make sense if it happens on steep downhills like OP states.

Good Luck:ride:

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