Are the plugs wet after you have cranked on it for awhile? If not work your way backwards. Can you see fuel spray out of jet. This still leave two problems restriction somewhere or diaphram/fuel pump not working properly. If your ski has a Mikuni Carb on it the is a little filter or screen in side the carb and I have worked on and own two stand ups and seem like every two years or so I must open the carbs up and clean this screen/filter.