Rich engines don't quit, especially right out of the blue like you're describing. If going richer on the needle won't help/fix it, you'll have to go through the rest of the fuel system? Regarding having the clunk coming out of the fuel on the down line, if everything else is OK, the engine will run for a good 10 seconds or so (at least?) on just the fuel in the line and carb?

Regarding which needle is causing the problem, if it's flying fine at higher throttle settings, I'd start with the LS needle?