This is what I would do to start with: Dump all the old gas, fill with fresh and use something like sea foam or a good guality system cleaner, check plugs, replace if needed, check filters(but if the air filter was bad, it would be chokeing it's self). I believe that would be a few things to start with. Gas goes bad now pretty quickly without something like sta-bil or a like product. Hope this helps get you goin again. It does sound right off, to me, like gummed up carbs and/or bad gas.