I run a 99 WR 400 with a FMF power core IV2 coupled with a FMF header pie (not Poer Bomb). I have removed the air box snorkle and the engine is YZ timed. Jetting is 48 pilot and 180 main. I am having a great deal of problems at the moment of which neither myself or 3 visits to 3 seperate bike shops can cure. This is my first four stroke so after 22 years on 2 strokes bear with me and my ignorance. My problems are listed below and any help/advice would be more than appreciated. The problems are as follows. 1) when reving the engine out of gear and holding the throttle at say 1/2 the engine pops and bacfires and will not hold a steady rev. 2) whilst riding when decelerating into a corner from a fast section, the engine back fires seriously (flames from exhaust and loud bang from air box). 3)The whole note of the engine is not sweet. These problems started whilst running totally stock bike, after a very very wet enduro (UK weather) and several good off's in rivers. The bike starts first time every time. HELP !!!