I have a '92 E350 5.8 with a Freight King box that has been customized for hauling my race cars.

As long as I've owned this vehicle it has been very slow to take fuel causing fuel pumps to click off even at their lowest setting. This year during my dewinterizing I decided to finally address this issue but I haven't exactly located the problem. I believe the tank may have been moved from its original location during the conversion.

I always assumed I had a clogged vent or possibly a restricted filler neck. I pulled the neck down this weekend to inspect and verified the vent is clear. I can blow compressed air in the filler and feel air flow through the vent. While performing this procedure I can hear a metallic clang every time I squeeze the trigger on my blower. I'm assuming there is a flap in the top of the tank which is causing my flow issue. I need some advice from those familiar with this fuel system before I drop the tank. Is the flap I hear in the tank or fill tube? Are there any problems with this flap that I can repair to restore flow? Can I remove it?

Thanks for any help. I just want to gather all the info I can before dropping the tank.