Steevo

I noticed some discussion about XS's here. I have an 84 Yamaha Maxim XS 400. My bike will start and idle fine, but as soon as I bring the RPM up over 3000 - 3500 the engine starts to bog down and sputter and it won't clear itself out no matter how much throttle you give it. It will do this wether it's sitting in neutral or you are driving it down the road. A buddy of mine has an 82 maxim so I tried the carbs and the gas tank off my bike on his bike and his bike will run fine. My bike has aftermarket exhaust on it but the bike was running fine with these pipes for a month or so (I've only owned the bike for a month) and then this happened for no particular reason. Any ideas?? Thanks, Steve