If the top of piston and the glow plug are a dark brown that is an indication that you are running the engine too lean. Richen it up a bit if, your engine is tuned right the top of the piston and the glow plug should have a slight tint of brown tho both of them. But it should never be a dark brown. Richen up the high end and low end needle's a 1/4 turn at a time until u see a good stream of blue exaust coming out the stinger at all times.. But all in all you ran the engine to lean.