I went to start my 50hp Mercury outboard over the week-end. It started and then shut do a few seconds later. I checked the bulb for firmness, it was fine, fuel gauge showed 3/4 and the cranking battery was fine. I gave the intake a shot of starting fluid, it started and shut down as soon as that fuel supply was exhausted. 
I have a clear fuel filter in-line and when I checked it, the gas was off color (very dark). It appeared that the fuel line was deteriorating, so I replaced all the fuel hoses and pumped the bulb until it was firm. It still wouldn’t start. The fuel in the filter looked okay, but when I checked it on Sunday, it was dark again. 
I’ve drained all the fuel from the engine and carbs, but I don’t want to push what could be bad gas back into it. 
is there a way to check the gas in the tank.  I’m thinking of hooking up a portable tank with fresh fuel  If it runs on fresh gas, I will know for sure the fuel is the problem. 
If so, should I replace the tank, or just drain it a partially fill to check things out? Thanks for any and all responses. 

First try the portable tank.  If it runs, drain or pump out the tank, add a can of gas treatment, fill it up and it should be fine.  2 stroke, 4 stroke, 3 cylinder, 4 cylinder?

1 hour ago, Alex from GA said:

  2 stroke, 4 stroke, 3 cylinder, 4 cylinder?

Its a 3 cylinder oil injection, 2 stroke