Just fixed a frieds 70 HP that was doing the same thing.
Get an electric fuel pump and pump out all of your old gasoline.
mix 2 cups of simple green into scalding hot water and pour it in the fuel tank.
let it sit for 2-3 days.
Pump all the water out.
add 1 gallon of Acetone to the fuel tank. It will remain in there when you fill the rest of the tank with fresh High Test Gas or AVGAs if you can get it.
Remove all of your all rubber hoses and any gas filters. throw them away and go buy brand new high PSI 3/8 fuel hose and ss clamps and a fuel filter bracket & spin on fuel water seperator. ( One for Each fuel tank) install them on the outside engine bulkhead so that if you spill and fuel it wont go into the bildge. Keep and extra set of 5 to 10 micron filters handy to cchange anytime your getting water in the fuel.
Remove the float bowl on your carbs and clean with carb cleaner. Add a small inline clear glass fuel filter under your motor cowling with a 2 mictron filter in it
Install new sparkplugs. The Platinum type with open electrodes
Tank up with fresh gas
Go fishing
