My friend has a Yamaha YFZ 450 ATV. Not more than 6 hours on it, brand new. All of a sudden, when you really slam the throttle, it spudders and back fires. Now if you ease the throttle on, it will accelerate normally all the way up.
What could be the problem here? Any ideas will be appreciated. http://www.gixxer.com/forums/images/graemlins/confused.gif

Its running too rich.
Either not getting enough air or getting too much fuel by the sounds of it.

Careful as you will foul up your plugs riding it like this

wrong fuel I would say ....

Thanks guys