hello fellow boaters. im haveing a problem with one cyclinder. i have carbon build up. every time i test the boat i run it at 4200 rpms for 5 to 10 min. and then all of a sudden the motor drops 1000 rpms. then i pull to the side and take off spark plug number one and see that its full of carbon. then i clean it put back on and run the boat for another 10 min and starts the same problem all over again. note that all other spark plugs are great. ive also used power tune spray and still continues the same problem. what eles can i do to fix this problem.