I have recently upgraded my cam in a fe 390 but now I seem to have a new problem. Low rpm and slow acceleration have suffered dramatically. I understand it will have a loping at idle as it is a couple steps above stock. There is a noticeable sucking noise like a vacuum leak that I can even hear in the cab while driving coming from the carb. Faster acceleration or higher rpms lessens the effect. And noticeable lag time if you just stomp on it. I run a Edlebrock 750 with manual choke.

 

Vacuum leak. You need to isolate it and fix it.