the biggest issue we now face is not the gasoline itself its actually the lack there of. todays "fuel" is mostly low grade petroleum, with lots of additives. Ethanol is an additive to "cut" the fuel, much like similac is used for cutting cocaine. All the refineries are doing is cutting the crappy quality fuel with other semi decent burning liquids and meeting a R.O.N. factor, or octane #.
Have you looked at fresh fuel in a clear jar? Its green/brownish and has a vile ammonia smell to it, to the touch it is oily and leaves a residue. Not many years ago it was clear/pink with a strong smell and it would evaporate rather quickly.
The quality has gone down but not the pricing in the last 30 years.
I worked in the marine/Marina industry for most of my life years as a technician There were always warnings of "oxygenated" and "ethanol" fuels.....they have been playing/adding with these mixtures since the early 80s, so its nothing new.
Stabil (which is mostly ethanol) was good until about 2006 when the ethanol was introduced largely and exclusively into all gasoline. The marina I worked for was forced to remove our "old fiberglass" tanks and install new ones to support the "new" fuel coming....its either that or don't pump gas anymore. Anyway, the ethanol was causing all kinds of major issues, blowing up new and old engines clogging injectors, mushing rubber needle tips and collapsing fuel lines etc. but that's for a different thread.
The lifespan of fuel has been depleted, making its octane hold just long enough to get from the refineries to your gas tank. Its not meant to be left in storage anymore in a gas can, mower, motorcycle, boat etc.
The issue is that ethanol absorbs moisture from the atmosphere, so vented tanks end up with stale fuel (varnish) and water (that will turn to rust) over a season of sitting. Non vented tanks just end up with varnish.
Seafoam is good but also not the endall answer to crappy fuel. It like any other fuel additive is just that.... an additive. Kind of what started this whole mess in the first place. (Remember the oil companies cutting the fuel with the ethanol additive?)
Any fuel stabilizer is meant to keep fresh fuel, fresher a little longer, not rejuvenate old fuel. That's impossible.
Here is my best advice I can give, and this is from personal experience on my own and many of my customers boat and generator systems...From the little 2.5h.p. 2 stroke kicker to the Yamaha v8 350h.p. 4 stroke outboards, including all the inboard motors that are "gasoline run" (This does not apply to diesels)
1. Run all your engines weekly... or if not possible
2. Keep your tanks as empty( NOT FULL) as possible when put in storage, spray or put fogging oil in the tanks directly. That's the hard part.
3. Carbs. disconnect fuel line, run the engine till it stalls, turn on choke and start engine, run it till it stalls again , then drain the bowls of fuel, fill fuel lines,( Pre fuel pump if it has one,) with fogging oil, crank engine over until bowls are full of oil. disconnect battery and put on trickle charge.
3.EFI....Do not run dry! Run the tank till its down low low, add 1/2 gallon of fresh premium gasoline with a heavy dose of your favorite additive (stabil/seafoam). run the engine for about a minute......or... purchase fogging oil that has Schrader valve threads (Johnson/evinrude...BRP sells it.) in its spray can. the J/E BRP can even comes a clear flexible hose making it easily to get to those pita valve locations. while spraying into the system run the engine for about 1 minute, that will be long enough to get the mixture in the tank into the rails and injectors. disconnect battery.
In the spring, Fill tank with fresh fuel, connect battery. and go.