So fuel tank issue was looked at but wasn't deemed to be the culprit as we ran a separate external fuel supply to the pump, and had the same issues.
Carbs are clean and idle screw behaves as expected. We also swapped another set of Webers on and again, the same result.
Running electronic ignition, this was the first thing I suspected, annoyingly bought a new one (at least I have a spare now anyway) but want't the issue. We swapped the dizzy out for another 050 which ran points and vacuum condenser, but alas didn't solve things!
Valves are all free and clearances checked.
No closer to a solution I'm afraid.
![Confused :?](./images/smilies/icon_confused.gif)