Hmm, here goes my guess.
If you remove your air filter cover and look inside the throat of your carb...how high up is your brass slide (the round barrel slide that your throttle cable connects to)?
About 1 - 1 1/2 years ago, NT (stock carbs with happy time engines) had a serious issue with the barrel slides sticking in the carb. They were not machined or polished enough and would not slide easily...they ended up getting stuck and creating a serious problem.
If your kit is new...contact your seller. He/she should give you a replacement, as they probably know about the problem.
If you don't want to wait on a replacement you could remove the brass barrel and polish it down with some emery cloth...just make sure that it is perfectly clean when you put it back in the carb, and that it will slide up and down without any force at all. You should be able to drop it in there and as long as the pin is lined up it should fall right to the bottom and you should be able to tip the carb and have the barrel fall out again.
When it slides that easily your polishing is done.
The only other thing that i can think of would be that your float valve is stuck, or you have your engine installed so you carb is sitting on one heck of an angle. Either of those thoughts are likely to cause the carb to keep dumping fuel into the engine. Correcting that should be relatively easy, and I'm sure that there is a post on the forum somewhere for carb issues.
Take care, and ride safe.