If your pump is run off of crank pressure then it's probably a Perry VP-30 pump. Perhaps you might try posting this question in the engine manufactures forum, they might be able to help you the most.

What happens when you turn the engine right side up, how does it run then?

BTW, the return line is a good idea. I was having a little trouble with over pressurizing the carb on my B1, I'll have to try that and see if that helps.

Thanks.