"Hold it! This is a common bo
"Hold it! This is a common boating problem; it has nothing to do with the ignition or a screwed up carb. The backfiring is caused by a lean out condition because the carbs are simply dry after sitting in the heated engine room--it happens all the time. Pull the carb top off after it sat for a week and you won't find a drop of gas in there.
IF this boat has electric fuel pumps, and IF the pumps run ONLY off an oil pressure switch (ON when pressure is evident), then you can do this: Add a push button by-pass switch to fill the carbs up before cranking the motor. I did this on my old boat and it worked slick.
But IF the boat has mechanical fuel pumps, then you have two choices: Install electric pumps (see above), or use the "Fastjeff Boat Leaving Method". To wit: Let the motors cool down all the way (if possible, opening the hatches to aid). Then, with the motors cool, refire and let the motors run only until oil pressure comes up. Next time you fire up there will be some gas remaining inthe carbs, and that will get them going.
Jeff"