Where is your exit ventilation hole? You have to have an exit hole larger than intake. This will make a negative pressure at the rear to suck the hot air out. Only having an intake will just create a pressure bubble inside and recirculate hot air.

Other than that if you are in fact running a bit lean, then after you get to half a tank or so the pressure may drop with it fighting gravity. If it's already lean it's going to lean even more.

I would cut an exit vent and try running just a bit more rich and see what it does.

Check the inside of the tank too, make sure your clunks are still attached. I don't think you should have a problem with this setup so check the clunk, cut a vent, and run a bit rich.