For the Black Widow try replacing the needle valve spring with a short length of silicone fuel tubing and a thin washer. The washer goes over the NV threads so you may need to carefully file it out a little. The fuel tubing offers the tension to keep the NV from turning from vibration just as the spring does. But it also acts as an air seal to prevent any air passing in through the treads in an uneven manner.

Other causes of uneven runs are often related to the reed. If it's one of the old copper ones then get a rebuild kit that uses a mylar reed. These last almost forever. Also on older reedies sometimes the little O ring around the intake manifold that runs down the center of the tank leaks and that will upset the run.

For control line stunting with Black Widows or similar engines you need to open the tank up and ensure the pickup tube runs over to the point at the back of the tank that is up the side towards the outside of the circle. Looking at the engine from the rear the pickup tube end should be wedged into the backplate at the 3 oclock position. The fill and overflow vents should be just up and down regardless what you do with the rest of the engine configuration otherwise the fuel will flow out of them and drain about 1/2 the tank before you get into the air. With the pickup as said you will only get 1/2 a run if you aren't flying but once in the air the centrifugal force pushes the fuel to the outside and you'll pick up at least 80% of it with the pickup at that position.

TeeDee's run great on bladders but they are a pain to work with. You might try only using a 3/4 oz tank at most and keep the tank as close to the engine as you can. It's impossible to have it too close. Under no circumstances will you get a nice run if the tank is further away than about 3/4 of an inch.

If you do try the bladder you can get thin wall latex tubing at fishing supply outlets and that tubing makes great bladders. The tubing blows up to a diameter about the size of a hotdog and gets long really fast so be sure to allow enough length in the tube in the outboard wing.

That red mount is all wrong for flying control line. Perhaps for a trainer it would work but I'll bet the fuel pickup location in there is all wrong for stunting. If you knew where the pickup was located in the tank perhaps you could cant the whole mount and engine over at an angle so the pickup was to the outside of the flight circle but it's a big lump of plastic to have on what is supposed to be a lean and trim little model. Better to use in internal tank or bladder.

For the uniflow tank it's all about the vents. I tried to find a good explanation on the web but there doen't seem to be one that's easy to find so you are stuck with mine...

The pic below shows a top view looking down onto a tank in flight. The blue line is the fues being pushed out to the outside by the centrifugla force. The red tube is the fuel pickup and the black is the Uniflow vent. There is also a second vent you need in order to let air out when you fill the tank up but you must cap that vent off and use the angled one shown.

I started to write a description of why this system offers consistent fuel flow but realized that I don't know why. But it works. THe engine with a tank like this will run pretty consistently from beginning to end. The tank still needs to be positioned correctly in relation to the engine though. That means centered on the carb position horizontally and close to the engine. Maybe someone else can say why the Uniflow works.