I bought a new 2012 WR450 2 weeks ago. I've ridden street bikes for the last 30 years but this is the first dirt bike that I've owned so I must confess that I don't know much about them. The first weekend I rode it stock and found it to be a complete dog. I then removed the throttle stop and installed a FMF Powercore 4 pipe and now it has all the power a new dirt rider like me needs. However, I now have 3 new problems: 1) It is hard to start. It usually takes several attempts to start whether the bike is cold or hot. 2) It won't idle for very long without dying. I have to keep my hand on the throttle to keep the engine running. 3) It frequently stalls when I'm riding it when the rpm drops close to idle speed even when the engine isn't under any load (like slowing for a stop or into a tight corner). It is quite inconvenient to be decelerating into a switchback and have the engine stall causing the back tire to skid. The dealer told me that the WR450 is tested with this muffler so my assumption was that I would not have to have the fuel injection remapped. True? Not true?? Until I get much more experience on the bike, I won't need a higher performance mapping. I bought the bike for trail and desert riding (I won't be using it on the track) so I need it to run well at lower rpm. That's way more important to me than additional power at the top end. Thank you for sharing your experience with me.