I have a 2009 YZ450 and I am having issues with the bike boiling the gas. After about 15 to 20 miles of riding the bike with begin to studdard while on the throttle. Stop, turn the bike off, and pull the gas cap and its like the tank was pressured. With the gas cap on you can see a steady stream of vapor exiting the gas cap breather hose. The bike is backfiring on the decel but is not losing coolant. The bike is basically stock except for the FMF exhaust. Also, the bike ran excellent for 90 hours, it has only been acting up for about the last 5 hours. So far here what I have done and nothing has solved my issue: Cleaned Carb Main Jet - 168 to 170 Pilot jet - 48 red needle on the 5th clip Fuel screw - out 2 turns (all of these were the JD jet kit recommendations) New Petcock New gas cap New top End (piston, rings, valves) heavier coolant cap 1.6 Any help would be appreciated.