Recently I have been restoring several older Cox Babe Bee and black tank .049's. The newer black tank only has a fill port, just 1 port? How do you know when this tank is full? And do I treat it differently? I'm having trouble keeping it running once in the air? I am using Cox fuel 25% that I ordered from Brodak. None of the motors continue to run steady once in the air? I have cleaned them.  Replaced the NV. Added new glow plug. And opened and cleaned the tanks...Still having trouble....Ideas? Thanks.

Hi Ronald,
I see you are posting in the vendors section. You will probably get a lot more attention from the 1/2A guys here in this forum: https://stunthanger.com/smf/12-a-building/

I'll try to get back here this evening with some answers for you. In the meantime, the best resource for Cox and 1/2A help is at another forum. I don't think Sparky will mind if I refer you there. Try this:
www.coxengineforum.com.

A log-in nag screen may pester you until you join, but there is a "Do not show again" button. And we have a guest forum too that will let you post freely.
Rusty

Ty's right, the venturi seal. Flat and round, not an o-ring. The head pressure seriously goes to hell when it becomes uncovered. If you do use an o-ring, be careful not to distort or hairline fracture the backplate.