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Bill,
I am in no way a Webber expert, but I am starting to learn and am limited in what I can offer. Based upon what I have read and heard from Jim Inglese, one of the things to rule out is binding or looseness in the linkage. I would suggest that with the engine off you manually move the linkage and look and feel for binding or looseness. Do the carbs open and close consistently at the same time. Do it several times, as sometimes it could be on and other times not. You might need to do some adjustments to the linkage. Do you have a top view of the linkage setup?
Jim
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