You could have caused a problem by pressure washing, but it really should be okay after the machine sits and dries. You said you have good spark, so I would for the moment, eliminate that as the problem. But then, I have never seen Sta-Bil go bad or turn milky either, but I would think that it does at some point. Was it gelled?
After you put the Sta-Bil in, did it run right at first? In other words, did it run right until the Sta-bil had time to get into the carbs? If that's the case, I would lean toward siphoning the tank empty, cleaning the carbs, and starting over. Try pulling your filter out of the tank to see what it looks like. Or pull the fuel line to the carbs and see how your fuel flow is. If the Sta-bil was gelled at all, it may have covered the filter.
I may be way off here. I have never seen this problem with a fuel stabilizer. I'm just trying to throw things out there for you to try.
Let us know what you find.
Keith