only 10 teeth? jesus, the stock one has 47 or 49 ( i cant remember off the top of my head) they must have meant - 10 teeth or something
if the back one was that small the bike would hardly move. did you notice this happening?
sounds like bullshit to me thogh, the 250 has no torque at low rpm , so a too-small rear sprocket would make it hard to launch, that could lead to plug fouling i guess, but fuel getting through = problem in the carbs. usually around the floats not shutting properly.