Pulsing action of the engine on the MAG carb's fuel block is what is creating pressure on the return side of the fuel system to the tank.
There is a pair of check valves - one at the overflow (sometimes you can see this when environmentally disasterous refueling results in fuel coming out of the side of the boat through a small hole), the other vent is near the fuel tank itself. If the check valves are bad, they let water IN the fuel system or do not let excess fuel vaopr vent OUT of the system.
You most likely have clogged fuel filters in the carbs.
Easy, but limited chance of success guide:
1. Clean out spin on fuel filter
2. Try running your boat on RES - this would point to a clogged fuel selector valve.
3. Remove the fuel selector valve and clean it out with carb cleaner. Blow it out with compressed air in the ON and RES positions.
4. Change to new spark plugs.
5. Pull spark plug wire from boot. Trim 1/4" off the tip of the wire. Reinstall to the boot and ziptie it back on.
More complicated, but high chance of success guide:
1. Remove all grey fuel hose (if you still have it) and replace with black automotive fuel hose. The grey stuff breaks down and clogs the carbs.
2. Remove the carbs and disassemble for cleaning the internal filters.
3. Better yet, plan for rebuilding the carbs
4. Clean spin on external filter, clean out fuel selector valve.
5. Pull fuel baffle from the tank and check for any obstructions on the intake screen.
Click the link on my sig below to see what the more complicated guide looks like in pictures!
