Normally it's a vacuum leak that causes these symptoms. They are usually non-obvious leaks.

But in your case, this is the biggest clue:
Quote:The idle mixture screw on a weber 32/36 should make hugely noticeable changes. Since adjusting the screw results in no changes, something major is wrong with the carburetor or carburetor installation.

Did it run good at one time, and this just recently started? Or has it always had this problem since you fitted the Weber?