Hi Danny,
I had the same problem a week or so ago. The under and over 8cc tank is not too easy to get hold of (at least I have had no luck so far). Take a look at the topic "Cox 8cc stunt tank". There were some useful suggestions there!
I just missed out on one on UK Ebay, only a few minutes ago. I think you will need to buy a Cox engine attached to the tank!

Regards,

Andrew.

          Dan, I 'm a bit confused here. I understand that you purchased a NIB Black Widow. Reason for being confused is this, the Black Widow had a metal backplate and also a plastic backplate. The metal had no filler pipes on the backplate. The plastic one from the 80's didn't have filler pipes either. The newer Estes engine did though. Cox prior to Estes ownership was producing the single fill pipe plastic backplates. These backplates have a small vent hole right next to the filler pipe. If your not careful while filling, a powerful stream of fuel can come right out of the small vent and it generally goes right into your eye. I've seen it go 24" high or better. Are you saying that your backplate has 2 filler pipes on it?  All the newer Black Widows came with the large tank without the stunt venting. I've tried to modify the backplates but had little success doing so. It may be worth a try though. I have thought about modifying a tank just as Bill discussed. If you have a Baby Bee metal backplate on hand with the two filler pipes, you may want to try it. The venturi hole may be sized for the Baby Bee and may need to be drilled out for performance, but it will run as is for a test. Put the backplate on and using (extra small) fuel tubing make a loop from one filler to the other going around the engine to reattach to the other filler pipe. Place a v shaped cut on the tubing on the underside of the engine. Pull the tubing off of the filler to fuel but place it back on once full and go fly. When the plane goes inverted, the v shaped cut will vent the tank allowing the engine to continue running. This worked for me using Baby Bee engines and .020's but I never tried it on the Black Widow tank. I can't see how it would change anything, I just never tried it. I had several repair and modifying issues with the plastic backplates which resulted in me throwing them out and purchasing new ones. If this does work for you, you may need to drill out the intake hole in the metal backplate . Don't go to large as fuel draw issues can result. I think the max is .093. Ken

Sad to say, in the end of days, Cox made fake Cox engines!  The "Killer Bee" was downgraded to the simple product engine, and the "Black Widow" wasn't set up for stunting.   

A true Killer Bee had a special crankshaft with better balance and stronger steel, a special cylinder with swirl porting, a drilled out venturi and Anodized crankcase.  One very visible clue is the plastic needle valve extender.  We had to shorten and lighten the needle to keep it from unscrewing due to vibration!  22,000 rpm can do that sort of thing!

The Black Widow SHOULD have had black anodized case and tank, twin bypass cylinder, over/under stunt vented tank and a .087 diameter venturi. (may not have the venturi dimension quite right)

I guess I will have to tear down one virgin BW and my running Venom and see what the dimensions really are.  I thought we enlarged the venturi on the Venom since it was not intended to maneuver.