Hello; I have a big (one gallon) bottle of hand cleaner, it's called Gojo with pumice. I just squirt a little of it on the engine and use the included brush to get it to the bottom of the fins and in every nook and cranny. I let it sit for a few minutes then take a toothbrush to it under the tap. This dissolves all the castor crap and leaves teh engine looking just like new.

Now I don't have to tell you to not allow the water or cleaner to get into the carb opening do I?

I haven't tried to shift burned on blackness with it, but I think it'll do that too with a little sxtra scrubbing.