Vintage Dirt Bike Q & A

I have a 2001 RM250 that was a factory bike used in the 2001 GNCC. It has a warner modified motor. I run 927 in it at a 32 ratio. It smokes sufficiently and smells great (I like the smell of 927). My only problem is that I get black goo coming out of the silencer. I am on the third packing since I bought it. I found it started leaking goo after about 8-10 hours of riding from the time I bought it, and about 8-10 after the first repacking.

I hadn't thought much about the goo, everyone told me that it's normal and that you have to repack every so often because of it. Lately, I have heard that a similar condition could occur if the right seal is bad. Is there an easy way to tell if the seal is bad. Also, if it is bad, how can you change the seal without tearing apart the case?