The above posts are both good advice. I used to be a small engine mechanic and I can't stress how uninformed most people are about fuel for their outboards. "During Spring comissioning..." tells me all I need to know right there. Taking the carb apart and cleaning it with an appropriate solvent is the first step. A carb rebuild kit with new seals and diaphrams is also required, then I'de check the rest of the fule system.
I can not stress enough that at the end of a season or any peopriod of none use over three months you need to put some fuel stabalizer in the tank with whatever fuel is in there and then run the engine dry after that. If you're worried about wear on start up there are products for that too or put a drop or two of oil in the cylinder.