Ah, this in here... [The crawlspace beneath the engine is barely large enough for two people, and Sorey has to point from a distance to show her exactly where he's talking about.] Do you see those moving parts in there? On every fourth rotation, the one all the way on the left catches. I checked for anything stuck in the way but there's nothing, all the way up the shaft. So I think it must be a mechanical error, but darned if I can figure out which part it is.