Racing promoter gave you some good advice. If you get faster after 2 to 3 laps, does you kart get faster in the turns? tires! Air pressure, Try adding al little more air. Weight Are you a little heaver than the rules dictate, Also your clutch can have an effect on starts also, Where does it engage? Is it in perfect condition. Last are you sure you are getting all you can out of your engine.
Fixing problems in racing is almost always trial and error, or process of elimination. One thing to never do, is to make more than One change at a time!