I just got a 01 YZ426 today and took it around the block a couple times just to see what the 4 stroke motocross bikes feel like. I notice that when I chop the throttle at high revs the engine braking causes some snaping and popping that sounds like backfiring. Is this normal or is this an indicator that I need a new spark plug? It does have an FMF slip on but a slock header. This thing is so obnoxiously loud to my neighboors but I love the sound. All I know is that this is one serious machine and I love it!!!!!!!