+1 on the carb bowl mung.
You don't mention the make/model of bike, but, assuming it's carb'd, the same answer applies.
You'll probably have to clean the carb(s) mechanically. SeaFoam, while great stuff, usually can't unclog jets.
When I worked at a boat shop, every spring saw a mess of folks bringing in their boats and/or motors, 'cause they'd (usually) start, but die when you put them in gear. Munged up carbs. Some of the "fuel" we'd drain outta the carbs and tanks would almost make you gag.
And I'm not even going to bring up water in the fuel. Oh, wait...
Next time you put the bike away for a nap, treat the fuel system (not just the tank) with a fuel stabilizer like StaBil. It's also available at Walmart. I use it in all my gas-powered machines.
Get 'r done, and get riding!