Sorry bro, I didn't know how long you had been playing with these little buggers, so just assumed you were fairly new

Without actually seeing and hearing the motor run, and having a "hands on" play, it's just too hard to surmise what the problem might be on-line.

I am told all the GO parts slip straight into the Nova +4, so the Nova carb should be a sure bet to fit the GO.
Let us know what happens when you fit that. Check the OR-0004 and OR-0014 O Rings when you pull the carb too. This can be another spot for tiny air leaks.