how much fuel have you run through it?

Mine done the same thing untill it was starting to run in then it was a matter of lean up the top end a touch fly then land and do it again untill it run perfect no stack on.
Open whole near the carb. 98oct fuel 100-1 amsoil.
The low needle needed an 1/8 turn richer.
It is a matter of getting them balanced i found the low needle on the thilsone carb will efect the top end asswell.
I spent a whole day on this to get it good again after trying the velocity stack once fitted and re-tuned I lost 400rpm but had more grunt down low but sacrificed my uplines going vertical. removed it LOL

They are fidely to get right but once you get it you will love it instant responce heaps of power no gurgling inverted