[screamingchief]

Maybe try it with a base with less of a drop (flat) instead of that drop base you have on there now,my thoughts say it'll work better.

Sounds like with the lid on it it's restricting the inlet air and affecting the secondary opening action or such,and possibly the fuel bowl venting as well.

Sounds like anything in the air path is obstructing the flow really.

It also would help if the pan were a little bigger and not so close to the sides of the filter element,as that cuts down on the flow in that area as well likely.

A simple test might be to try a taller filter element if you have the room.

Just a few ideas.