I`ve got the exact same problem with my Keihin CR33`s, carbs were supplied by Steve Walls from Gremlin Racetech & he swore by them, they`ve only done around 110 miles since being fitted & as I stupidly left them with fuel in I`ve had them apart a coupe of times to to make sure they`re absolutely spotless but has made no difference. Thinking of replacing with Mikuni 36`s as they seem to be a bit less temperamental.
Bike is a 79 GS1000EN with 1100 Wiseco kit, Fast Road Cams, Crank by Chris Applebee & Head by Nick Stratton.