Copied the following from another forum:
There is lots of discussion about this in the boating forums. If you put it in and run it right away, you won’t have any problems. The problems occur when you have to leave it in your tank. Alcohol in gas is hygroscopic - it attracts and blends with water. As your tank breathes in and out from heating and cooling each day, water (humidity) goes in but gets attached to the alcohol molecules in the gas and stays. After a point, it gets saturated and you get “phase separation”. The water/alcohol mixture falls to the bottom in large quantities and when you suck that through your carbs BAD things start to happen. A water filter will help, but unless changed very frequently, will quickly be overcome with the shear volume of crap in your tank.
There are two ways to cope:
1. Keep the tank absolutely full. A full tank has no room to breathe therefore less chance of water being absorbed. The downside of that is that if you don’t use it enough the gas will go bad. Ethanol blended gas goes bad in as little as 60 days.
2. Keep the tank absolutely empty. After each trip, totally empty the tank and use the gas elsewhere (I run the pre-mix in my lawn tractor - smokes a little but no other bad effects). Before the next trip, add about a half gallon of gas to the tank and manually pump it through the filter. Empty the water separator before and after doing this. Then add as much gas as you think you’ll safely need for that trip. Empty it out when you are done.
I’m doing the latter method. I disconnect the hose from the motor and drop it over the back of the transom into a gas can and use the bulb to pump it out. Usually, I can start a good siphon and it will run on its own.