So my 93xp has been running great then all of a sudden im going down the lake at WOT for about 3 miles and it dies on me about 1/2 mile from the launch....so i swim it there get there and it fires up....get home pull the plugs and the rear is black while the front is light brown...cleaned the plugs up since they were new that day put them back in fired right up brought it to the water fired right up rode for about 30min and it started sputtering....pulled the plugs and the rear was black again. Any ideas why only the rear would be doing this?