The weather turned rather nice in the afternoon, so I took a quick spin. It started fine, and idled fine. But first I noticed misfire at high rpm (beyond 6000-6500), and it got progressively worse, and at near my house, I couldn't rev it passed 4500rpm without the engine missing and backfiring. Pulled the plugs, and they are black-ish (not wet but not sooty either, just black-ish than fox orange that I normally associate with the healthiest plugs), but it always ran a bit rich, and from the condition of the spark plugs, it doesn't look particularly any different from last year when it was running really well (till the electrical problem, which I fixed over the winter). Anyway, what's baffling me is that I took quick 85-mile joy ride just 9 days ago, and it ran perfectly at the time (and I had a very enjoyable ride). Prior to that ride, the carbs were gone through again, and, oh, now I think about it, the plugs were brand new then. So I don't know what happened between then and today, and I don't know where to begin. Only mechanical change that I made in between was rear sprocket swap, and I know that has nothing to do with it. Hence, I don't know where to begin. The engine is stock, with a bit over 21000 miles, it's got slip on (with db killer), stock carbs with Dynojet kit and open airbox with K&N filter (that's how it came when I bought it). Could anyone give me a direction/suggestion?