ORIGINAL: fly boy2
I dont know the name of the part but its the big round thing, first thing you see when you take the backplate off, looks like a counterweight, that was the only thing with this rust looking stuff on it, I'm using wildcat 15%, I run a lil rich since the engine can run off of fuel iwith castor oil but I have to run a lil richer than normal
It sounds like you are talking about the crankshaft and some of them are in fact counterbalanced. Whether it is actually rust there or stains form other stuff, it likely is no big deal. I'd run it and have fun.