Hi. Recently tried starting my S3 after 7 years. New electronic ignition. Valve clearance set, timing roughly TDC. Tried both the current Weber ICH34 carb and an OEM zenith type carb. It cranks but won't fire. Good spark at each plug and fuel reaching the carb. Violent backfire in the exhaust every so often. Struggling for ideas so any help gratefully received - Thanks

Sounds like you may have put the electric ignition in 180 degrees out, first place to check

Agreed - nowt that can really go wrong. If you’ve got air, fuel and a spark it’ll fire unless the timing is woefully out (180 degrees). If the timings correct it will still run (might be a bit ****e). Backfires normally mean timing massively out.