OK..

By "feel" there is a little looseness in the throttle valve
bushings in the carb.. we gona look further.

Thanks to all for some great thoughts/solutions.

Can anybody suggest some tests to confirm an intake system
leak that allows excess low DA air to lean the mix when the throttle
valve closes rapidly at high engine RPM?

What clearance should there be in the Throttle Valve bushings?
(if any)

Dave