Oh shucks, now that isn't going to plan. I was convinced you'd found the problem when you posted the pictures and we could see how difficult it was to bolt the manifold securely.
The soap solution idea is a good one for finding leaks, I think I'd modify it slightly and use neat washing up liquid applied with a paint brush around the manifold although it there's a leak underneath then it's a tough job to see it.
On a positive note, I still reckon it's going to be something simple in the end, and at least the engine is running ! I'll scribble a few random thoughts which might spark an idea somewhere....
most webers I've seen have very hefty return springs to force the throttle closed so if yours doesn't, there's one option. You've got the cable disconnected, try unscrewing the idle speed screw completely and then pushing the throttle closed to see if it drops the speed.
if you shut down fuel flow completely with the idle mixture adjustment screw then it would make me look elsewhere in the carb settings. Air leaks are the classic answer to elevated idle speeds but even so you need some petrol - it won't run at 4k on air alone
I could see the float height together with a sticking throttle causing the problem by feeding in a rich mixture but that's a long shot. On another tack, what sort of flexible mount do you have there, is it a rubber block or a metal insert holding rubber o-rings ? I did get an O-ring twisted out of the holder once and that cause a problem on that inlet tract as you might expect - easily done and difficult to spot until you pulled it apart.
Do you have something like a colortune spark plug ? If so, that would instantly let you know if the problem was a rich or weak mixture and if it was on one or both sides of the inlet manifold.
As I said, random thoughts (some of which will be rubbish
) and I don't have a solution, but a discussion might throw up something else to look at.
Brian
