If the SU carb has an air leak, it will create a lean condition for which no amount of tuning will correct.
The problem with the SU carburetors is that the throttle shaft becomes worn and allows air to seep past the shaft and into the carb causing a lean condition.
You can try contacting ZTherapy who specializes in SU carbs and parts. Also they sell a video called "Just SUs" it explains how to repair and tune the SU carb.
Just SUs video covers: 4 hrs of advanced college level theory of operation, diagnostics, tuning, surgical Weber removing and disposal. Covers all years of Japanese Hitachi "SU" and 1.6L-2.0 Volvo HS6 1 3/4" SU (HS4, HS2 same). Our newest video, 4 years in the making. Directly replaces the first 1.5 hrs of ZTV01. Heavily updates and corrects info from ZTV01. Part one is inspecting and installing new ZT carbs and "Tuning for Dummies".
Part two is "Installing and Tuning a ZT Master Tuneup Kit for Dummies". Balance of tape is answering common questions, racer tech and mods, airflow studies, horns, cold air box, etc. We address the Q+A sessions from the 2000 MSA and LSV shows-I worked hard to answer your questions on this video. If this does not answer every issue you ever had about the mighty SU, call me! If you still don't get a satisfactory answer, demand your money back!!! Ships free with new ZT carbs and the ZT MasterTuneUp kits. Available in single play mode on two 2 hr SP tapes for $20 or on one single EP mode tape for $15 (lower video quality). DVD available for $25