i have a 87 honda 4 stroke 7.5 hp. the problem is when i run the engine at the lake the exhaust comes up in the cowl. the engine runs like crap with the cowl on but runs great with it off. the person that had before me cut holes in the cowl to get fresh air in. how can this be fixed? hte local boat shop says it sounds like a cracked exhaust tube. i have a manual but doesn't show anything about the exhaust.