I have noticed that some engines do in fact smell different. I have always blamed the quality of the materials used for manufacture of the particular engine as my fuel has been consistent. The engines that 'smelled' were labeled "cheaper metal" by my Dad and I. We also felt those engines needed more oil than the one's we considered thoroughbreds. That may not find people that agree, but it was our observation through several decades of running glow engines. It is much less noticeable with the more current ABC/ ABN types.