Throttle shaft wear

What is an acceptable throttle shaft/bore clearance? I have a '64 4100 and the secondary shaft has about .004" of clearance in its bore. The shaft turns smoothly unless the engine, a 390, is running. Then the secondaries won't open because the shaft is too difficult to turn, even manually. Turn off the engine and it's smooth as silk. Restart and the shaft sticks. I've had the secondaries disconnected for the last 20 years because my wife was too fond of "that wooshing sound." Now they don't work. I've rebuilt and cleaned the carb obsessively and I know clean. Any thoughts?