Current Projects chat and miscellany

Trying to get the rear mech set up properly on the Viner, not set up downtube shifters for a while.
Can't get the chain to stay in the largest four sprockets on a six speed block, cable tension seems to slacken off and the chain drops into the two smallest sprockets.
Symptom of too much cable tension ? Cable is fairly slack in the smallest sprocket though. Anything else that could be causing it ?

To answer my own question if anyone is interested, it appears as though it's the friction shifter that's at fault - Not having enough friction to keep the cable tensioned.
Hopefully it's just the bolt that needs tightening. Otherwise it's take it apart and clean, or worst case there's a part that's missing, worn or incorrectly installed. Time to try to find some exploded Campag friction shifter diagrams