Have a 1999 Bayliner 3055 w/twin 5.7 Mercruiser. One engine backfires thru carb at 3000 rpm. I had carb rebuilt, replaced fuel filter, put on new dist cap and rotor, new coil, new platinum plugs, still the same. I had a friend in the engine room while running and making it backfire, he removed plug wires one a time untill it stopped backfire. Number 3 cylinder plug wire off and it ran on up and did ok. We thought maybe a valve... Others say NO WAY. Help