Gently play around with the throttle and be sensitive with it. Always remember slow in, fast out method. A good practice is to get used to drive slick tyre kart in a wet condition even its almost impossible to drive. Once you master them, driving with wet tyre would be just a piece of cake.
Another tips on the wet is try to shift your body weight is to lean slight forward when understeering (not too much) turn and lean slight back when exit. This requires the rear end gives more bite and grip when lean back. This requires a lot of practice.
*This rainy season would be a good choice to practice wet techniques. CityKarting in Shah Alam close at 10pm.
Driving in the wet require different techniques then your normal usual driving.
Brake earlier, spend less time on the brakes, tap it dont hold too long or else the kart would understeer like mad, counter the steering before the entry of course, try to avoid race line (not really necessary but depending on the surface situation).
When apex and exit, be gentle on steering and throttle...
On the contrary what u should do is on entry into the corner u flick the steering wheel full lock to the direction of the corner while hard on the brakes.
That will sort out the understeer issue.
By doing so becuase of the castor will allow the rear inside wheel to jack/ lift up and allows u to turn into the corner.
Remember it understeers on entry is because u have a solid rear axle.
The premier kart event in Malaysia for this year is the Rotax Max Championship. 1st round is held in Kelantan and the rest in Sepang.
I would suggest to rent a race kart first before comitting big bucks by investing into a chassis, engine to see if u're competitive enough and willing to spend the money.
Cheers over and out.