kawasaki sx 650 not running rite?

[Brian]

just bought a 1989 sx 650 took it out first time on the lake and it broke down took it to a shop and they wanted 350 to put a carb rebuild kit on it i said no took it home and took apart the carb got it starting and running no problem i went and took it on the lake today and it started out very week maybe 10 mph drove it about half way across the lake and it started picking up and seemed to run good then the motor slowed back down and the ski started going slow again. every time u start it it wont take off quick but after about 2 minuts runs better even goes fast sometimes very inconsistent i have a carb rebuild kit im getting but not sure that will solve my problem