I would start out with a couple cans of spray carb cleaner. Spray the shit out of all the linkages and little moving parts, they are most likely all gunked up from 15 years of grime. Make sure the carbs are supper clean, does not hurt to spray down the inside of the carbs too while the motor is running. You will have to rev the throddle while spraying down in the carbs so it does not stall. Then lube up the all the little linkages and suff with a good lubricant (not wd40) and make sure everything moves freely. Make sure all of the electrical connections to the carb are making good contact. Check for any busted vacume lines and stuff.
Thats about it, if that does not work, maybe you will have to fix up a manual choke cable or something.