Stating the obvious id start with the spark plugs, I had a bike ages ago that lived outside and after some particularly foul weather it ran like a sack of crap, turned out the rubber caps that seal the sparkplug hole wasn't sealing and the spark plug holes were half full of water, I sucked the water out with a Henry Hoover (yes I know Henry isn't a wet and dry but Iv yet to kill him using him as one) then took plugs out and cleaned them, let the leads dry and sprayed lotsa WD40 in and it run top notch after.
