my friends revo has been having this same problems trying to lean it out and richen it, that black fluid has now spread all over his chassis which he has to clean it 3 times a week. It might be something w/ your piston I keep telling my friend I think his is warped, because when he first got his the 2nd day he had it about his 9th tank of gas he showed me it and he didnt have a temp guage so I took mine and it read 399 degrees (400 pratically). Right behind his engine where the pipe is you can feel warm air coming out, at first we thought it was the header that he replaced because it broke, and he got a new cooling head. He's finally able to run it without it shutting off tht soon on him its now about every 5 minutes. When you run your engine that hot it can warp tht piston pretty ez, which can crack your engine because its now having tht perfect fit inside the cylinder and may cause tht black crap on your chassis thts wht I believe my friends problem is but he is very stubborn and still refuses any of my help. I would take yours into your lhs and have them check it out and see what they suggest in doing.

Mako3