1) assuming that you have the original vacuum retard distributor - disconnect the vacuum retard and plug the hose from the manifold. Try it like that, and then try it with more mechanical advance. The car should really have a vacuum advance distributor as they had everywhere except the US. Your distributor bushings may also be worn, throwing off the timing.
If the rotor and shaft have any side to side play, then the bushings are wornout.
2) Assuming that its a lean stumble, then the only easy way to richen the mixture ( and the lean condition could be caused by any number of factors in the FI system, its wiring and sensors ), try increasing the fuel pressure.
As to carbs on a supercharger and altitude - its no more of a problem then on a normally aspirated engine with carbs.
They do not adjust automatically to large changes in altitude, while a modern FI system will.
Hope this helps.
Regards,
John
Vintage Performance Developments
