Cusser
+1y
An issue is that vacuum leaks can occur with Weber installations if you don't "sand" the adapters flat using wet/dry on a sheet of glass. And vacuum leaks can affect your timing big time when you try to set it at 900 rpm.
Either find that vacuum leak and fix, or "band-aid" and set the timing to 6BTDC with the vacuum line unplugged from the distributor and the vacuum line plugged, then re-attach.