I bought this bike for my 15-year old daughter to learn to ride and we puttered around on it this weekend where she learned to start it up, get going from a dead stop in first and shift (just first to second and back down). Unfortunately, the bike smoked like an old stove burning wet wood at everything but low idle where it ran fine. Any idea what might be going on? I changed the plug at the beginning of the season and I don't think it has anymore than 3 hours on since. Thanks for any input in advance. Kent P.S. The bike has done this only on and off but I chalked it up to rich oil mix in the fuel. It wasn't doing it during the previous ride and I'm using the same jug of oil/gas mix as before.