Freddy and Mike, it does sound like the kart is binding up. The give-away to me is the rain really makes things slick, which frees up the chassis. The other give-away is you’re running a lower HP engine (206 vs. the two-stroke).
You definitely have to free the kart up to make it roll easily. If you have a 50mm rear axle, change it to a 40mm. Go with short hubs front and rear. If you’re running any seat struts, take them off. If you’re allowed, run a harder compound tire.
Depending on your kart type (Euro versus American), widen/narrow front and rear tracks to take out grip. Change your torsion bars (if you’re running any) to whatever compound gives you the least grip/chassis flex or take out completely. Like Steve said above, don’t run the floorpan loose (loose allows the chassis to flex).
Again, the issue is the kart is getting too much grip and the lower HP engine is having trouble pulling it. You’re gonna have to “unstick” it!