So if you bought a new torque converter, I doubt very much that the spring is too aggressive and holding it closed. Especially since you say with the wheels off of the ground it works fine. Here is what I think your problem is. Looking at the first picture in post # 1, it looks like your axle sprocket is way too small. This creates a very tall ( high ) gear situation. The engine probably doesn't have the torque
(guts) to spin it fast enough for it to open when the kart is on the ground with you sitting in it. Very different load situation. The clutch has to spin fast enough for it to open. You need a bigger sprocket on the axle. I understand this can be difficult and sometimes expensive. A cheaper and faster way to help would be to replace the 10 tooth sprocket ( probably want it came with ) with a 8 tooth sprocket on the torque converter. This will lower the gear ratio by 20%. Enough to let the engine wind up faster, spinning the driven faster, and giving it a chance to open.
