[RidgeRunner]

If it were the fuel pump you ought to be able to mash the primer ball hard enough to fill the fuel bowls and keep it going. I lean toward fuel because it died off when you decreased the fast idle. My motor did something similar a little while back. It would idle for a while and shut off but the primer ball didn't feel right. It was kinda squishy and never felt firm like it does when the needle is in the seat. I didn't discover the primer ball was bad until after I took the fuel pump off inspected it and put it all back together. Went to the local marina and purchased a primer ball, installed it in minutes and cured the issue. The old one had a faulty check valve even though it would still move some gas when tested. I left the old one in the splashwell, it hardened up like a piece of plastic. I say ethanol finished it off as it was nearly 10 years old.
Hope you don't mind but I got a question for the gurus about your motor. Assuming all the jetting is the same in the carbs wouldn't having one side lower by 20 psi cause a bit of a nightmare on tuning it to idle smooth? Is there idle air adjustments or something?