My bro and i just had to bring our 1991 40HP Mariner for electrical repair – the Stator and Switch Board – to Supreme Marine in Minneapolis. Because it was sputtering, we thought it was fuel delivery issues and we replaced the fuel line, connectors (tank to motor), and sparkplugs. We also told them we use a couple ounces of seafoam per gallon on every other tank.
Perhaps it was the age of our motor but Supreme Marine experts recommended we avoid using Seafoam. As they explained, part of the reason was because Seafoam has such a high alcohol content that it does burn and break away build up, however it also dries out the gaskets and other fittings it touches. The result can be cracked seals of the sorts (I cant speak to which ones per-se however i see where they are coming from.)
Instead, Supreme Marine highly recommended to:
1) Stay away from Blue Planet gas as that is notoriously high in other detergents harmful to boat motors (along with the ethanol content) — it’s manufactured for car engines. “Lucky” (insert sarcasm) for us in Minnesota, Holiday gas stations are all over the place. I like everything else about Holidays though…
2) Purchase ethanol treatment additives that essentially breakdown the ethanol/water component in the fuel. Depending on motor use, one $20 bottle could get you through your season. Examples are Star brite Star Tron Gasoline Additive (Star Tron Site with Ethanol Background), Marine Formula STA-BIL (Sta-Bil Site),there may be others but these are more well known/available.
3) For gas stations – ethanol-free gas is best as some have already indicated. I found this site that lists ethanol-free stations by state (Pure-Gas.org). If we can’t find the ethanol-free fuel, Supreme Marine recommended to gas up at BP’s, Mobils, Citgos, KwikTrips as there’s have less of these detergents. SA’s determined to be “ok” but not as desired as the stations just listed.
I suggest you contact your local marine motor specialists to confirm/corroborate what I learned from these guys. With respect to the comments already made, perhaps a motorhead can weigh-in on this thread.