4100 secondarys not closing

I have just rebuilt a 4100 [C6AF-L] and am using it on my 292 Y block .It has the big valve heads ,an Isky cam, headers and Dura spark ignition. The carb was from a 352, but I think it will work fine on my smaller motor. I followed all the specs for the 66 -352 and it runs very well until the secondarys open.Then they won't close all the way.I can manually shut them ,or shut the engine off and they will close.With the engine running the sec. shaft is quite hard to turn in either direction.Engine off and the shaft open and shuts very easy;no binding on anything.The vac. passage/restriction is clear,but it seems that vac. is what is keeping them from fully closing. Any ideas? I went back 100 pages but didn't see this problem. Thanks.