I'm having a hard time priming. It seems, after I prime it, air backs in from the carb and sits at the top of the tube. When I throttle up, most of them come out, but a few linger. When it comes back to idle, these could starve the engine momentarily.

I think this is what it is. Now the air may not be the problem at all.

(BTW, I tried jaka's suggestion, but it ran lean and the temp of the engine skyrocketed. I won't do that anymore.)