My 350 in my 66 C-10 was fired up today. I just rebuilt it. Fired right up, broke in the cam & set the timing but it seems to surge up & down in RPM's. Not really like a misfire but a slower surge, like a vacuum leak or something. I checked all the vacuume lines, manifold, & carb base but I cant really find anything. It really rev's up great out of idle. If I cup my hand over the carb it idles up I smooths out....I'm just sure its in the carb. Any suggestions??

Thanks