I've finished a bike restoration today and it's my first time using downtube shifters. They work and will span the full range of gears but won't stay in the big cog on the back for long, the shifter itself gets pulled down by the cable tension, I've tried loosening the cable but I'm just not sure how to set it up. Which gear should the bike be in, what position should the shifter be in and how tight do I pull the cable? I've tried tightening the actual shifter against the frame, that didn't help. It was a little D ring that pulls out, it's pretty stubborn so I grabbed it with pliers and managed to bend it off, ghetto repaired for now. I'm assuming it's cable tension.
The H and L screws are ok, I'll try disassembling it if I can find a diagram for my shifter.