I would not use E15 in any marine engine, in fact marine engine companies have said this. Also I'd use higher than 87 octane, use at least 89. The problem with E gas in general is that the ethanol absorbs moisture in the air and especially on older boats with open fuel tank vents, it can lead to the water and ethanol separating out from the fuel leaving water in the bottom of the tank with the ethanol and the rest of the tank will have low octane gas that doesn't burn well in a marine engine. After my problem last year I had the tank pumped out by a guy that does this for all the marinas around here and filled it with 30 gallons of 94 octane Sunoco straight gas. I ran this gas in the outboard tank test I did last September and the boat ran great on it. There are only 2 gas stations in all of Long Island NY that I know of that sell straight gas with no ethanol. All the gas docks sell E10 fuels. In the future I may just have whatever is left in the tank drained a the end of the season to avoid a recurrence. Now I used E fuels for at least 15 years and didn't have this problem, but it wasted a lot of time for me because I didn't think of the fuel as the problem.
Have you removed the fuel filter and dumped out the contents into a clear jar? Let it sit for a day or 2 and see what it looks like. If you see a layer of water on the bottom, that's phase separation.
