just when i thought that i finally had a decent, well-riding moped on my hands, my '82 Free Spirit just starts to freak out. so here's the scenario: it starts up just great and everything is fine. then after about 100 or so yards it starts to sputter and backfire. i thought it was the carb at first so i cleaned that thoroughly, and the problem went away for a little bit longer. but after a second test drive, the sputtering, bucking, and backfiring were back with a vengance. i checked the spark plug, which was brand new as of a week ago, and it was all black and sooty. i looked at that spark plug page in the resource section and it looks closest to being either carbon or oil fouled. i put yet another new spark plug in, and checked the air/fuel mixture, but after just one ride up and down the street, it was all black and greasy. the second spark plug looked like it was covered with something different than the first, though. i'm not sure if this is because the first plug had time to dry up and get all crusty, or if the substances on the plugs really are two different things. please help, i like this moped a lot. thanks!