I tried another kind of fuel tube. Unfortunately, this leaked too. I eventually went back to the hobby shop and got some black neoprene fuel tube, and that stuff worked well.
In the mean time, I had been testing the needle valve. While the leaks were happening, the needle valve was also working! I was able to reliably control the mixture that fed the engine.
One thing I had a hard time with though was throttle response. This first setup with the needle valve out of the subframe had long tubes, which killed throttle response. Fuel delivery in these small carbs is accomplished by gravity feed and suction from the venturi. I figured out that if I shortened the fuel loop, I could fix it.
When I fixed the leak, I also shortened the tubes and cleaned up the design. The wire sticking out from the valve allows me to turn it without reaching under the engine. If I ride slow, I can change my mixture while rolling.
As you saw in one of my earlier posts, I ended up trading for a different needle valve later on. It was just a nicer valve. I put a lot of miles on this particular valve and had no troubles.
I did have a couple of soft seizes when I was learning about tuning, but that was my fault, not caused by the needle valve. Same thing could have happened with jets.