The deal is, I was riding this past weekend and ran upon another XR400R that sounded much better than mine. After talking to the guy, I found that I could gain a pretty good little jump in hp (as well as get a better sound) by applying an after market muffler. So, here in lies the questions: -Can I replace just the muffler to see a boost in hp? -What are some mufflers that you would recommend? -Can someone explain a bit about jetting? -Is jetting necessary on the XR after replacing the muffler? -How much should I expect to spend? -Do I need an after market header or is the muffler enough? -What about spark arrestors? Sorry to beat you down with so many questions but, as you can see by my profile, I'm a newbie Thanks to everyone for any help you offer.