On my 426 I get a hesitation when I juice it hard.... Then when letting off throttle in gear I get some back firing... Bike has little hours on it as I bought it , then moved to different state, then opened a business etc etc. so my bike may have 30 hours on the motor. Any ideas? This is my first 4 stroke MX bike, before this bike it has always been 2 strokes, so Im a novice to this style of bike. Thanks for any input.