I have 90 octane marine fuel in my boat. Been in there 2 years. Started it today and port runs, but really rough and starboard wont start. When I smell the throttle body on starboard, it sure smells sweet like bad gas. I still need to check for spark but it is trying to start.... They are 496 based engines with 555ECU

How long does marine 90 last?
Do I have to worry about it varnishing and messing up the injectors?

1 year. Not great but still should run. Won't really hurt the injectors. Fresh fuel and give it 15 minutes it should come around

Did you add stabilizer when you filled it up 2 years ago?

No

Should I add new or get rid of the old and put in new?

The old fuel wont varnish or make the injectors sticky? Do they need to be cleaned? I have a fuel system cleaning tank, I was going to run a can of BG through both engines.

I would drain the tanks and put fresh fuel in, with stablizer this time if it going to sit. Should change the fuel filters also. Might not hurt to run fuel injection cleaner, Mopar combustion chamber cleaner, some type of magic potion to at least make you feel warm and fuzzy lol!

If it doesn't run better than who knows what it could be, things happen when motorized toys sit. Obviously could be fuel related but wouldn't be surprised if something else was wrong.

Buy 6 new fuel filters, 3 for each side. Start/run the engines for about 5 mins., change the filters, repeat. When the engines run smooth you can leave the filters for a day or so then change again. If fuel filters get too much water they will pass it through. I had wintered my outboard boat 1/2 full without full dose Sta-Bil for 6 mos. then went to do a Poker Run and neither engine would get over 2000 rpm. One would barely idle. Had all the guests, ice chests, etc; embarrassing. Did the above procedure and was back on the water before the rest of them had finished their breakfast tacos and bloody marys . Filled up with fresh 93 and the boat ran great all day.

Drain the tank. Run for 15 minutes. Then change the filters. You even open the cool fuel and suck the bad out of there and see the lower filter pad for crystals and water. Old fuel will not hurt injectors. Particles and fuel crystals will clog them up and possibly score the bores. Just swap the fuel and don't sweat it

would not run a marine engine on 2 year old gas even with stabilizer in it. All stabilizers say "GOOD UP TO A YEAR " when added to current fuels. And very few will claim good up to 2 years.

Keep in mind fuel RVP is important as well. Winter blend fuel used in the summer is bad and summer blend fuel ran in the winter is bad. Florida does not change their season blends as much as other states because the climate is more constant but that depends on south FL to N FL. Hawaii is the only State that has one blend of fuel all year round.

How do you dispose of the old fuel? Guess I could run small quantities through the wife’s lease car but larger amounts or leaded??