new/used bike issues

I bought a 99 YZF 400 in april. This is the first bike I have owned since the early 80's. This is my first 4 stroke. I have read and read the threads on here and have gained alot of valuable information. I have one question that is bothering me. The previous owner said, "it is a great machine. and has all the mods available done to it." I thought to my self, "ok." Not knowing what the hell he was talking about I was just happy my wife would let me ride again. :D Anyways, after reading as much as I could on Thumper Talk, I have found that I am having a problem with the "bog" that alot of folks talk about with the YZF's. The bike has a powercore 4 pipe on it. I would like to think that the previous owner would have rejetted the bike for the new pipe but I am not sure. When I got the bike he had a fresh plug in it. I rode it around for awhile...GOOD TIMES......... :D and then pulled the plug. It was a light brown and the bike seemed to run great. I am running 93 octane gas and live in San Antonio, Texas. I have put maybe 6 or 7 hours on it and I replaced the clutch and throttle cables and decided to check the plug while I was in there and it was BLACK. Not sooty just BLACK. Is there a way to tell if the bike has the correct jets that, as I was told, come with the new pipe when it is bought. I would really like to think that maybe it is something simple to fix/replace. Sorry for the long thread but I really like this bike and really enjoy doing the mechanic work myself so I try and learn as much as I can. Any help is greatly appreciated......

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