When we built ours, we used an actual Comet torque converter. I am not sure how the chinese knock-offs work but even with the governor on, we went from the original top speed of 18 mph to 30 mph and that was leaving the governer in place. If you have the converter cover off, look at what is going on with the CVT system and make sure that the variator is raising the belt up noticeably as it speeds up. If not, then something is going on with it.
Here is what ours looked like in proper working form: