Sticking throttle

Thanks Don.
I have what I need to build a 650 cfm flow bench and plan to build one this weekend. It will save time debugging my problem. I agree that vacuum at idle, combined with force from the secondary diaphragm spring, will keep the secondaries closed. Two symptoms suggest a sticking shaft.

First, the nylon arm that pushes on the secondary linkage bows under the strain. So the new diaphragm is working well enough.

Second, on a long stretch of highway I was able to get the secondaries open slightly. (The hood is off the car because I just finished an engine rebuild.) I fastened a wire “flag” to the nylon arm so I can watch to see if it’s moving. After stopping, the car idled at about 1200 rpm because the secondary throttle was not fully closed. I had to manually close it to allow the car to idle properly. Vacuum and spring force weren’t enough to close the secondaries.
Bert