I am fairly new to TIG and have been having quite a frustrating time trying to practice on my 250EX. I thought my troubles were just inexperience, however it was suggested to me that the tungsten turning black is a sign of not enough shielding gas. I tried bumping up the post flow, but at this point I don't think gas is coming out through the end of the torch.
After a lot of double checking connections, I finally tore apart the torch body itself to do some inspection. I disconnected the high pressure air line (the one shown in the attached photo), hit the pedal, and saw that there was a good flow of gas coming out of the hose with my regulator set to 10L/min. I then put my thumb over the end of the hose, hit the pedal, watched the flow go to 0, and released my thumb to find a woosh of pressurized gas come out. This lead me to believe that the gas system from my bottle to the end of the hose was tight and not the source of my issues.
I also tried attaching the back cap onto the torch, with the high pressure line still disconnected, and blowing backwards through the torch head. It's moderately difficult, but still flows a tiny bit.
Lastly, I reconnected the high pressure line to the torch head and threw the whole torch head assembly (except electronics) in a bucket of water. I didn't see any bubbles escaping from the hose connection nor the end of the torch were the gas is supposed to come out even after multiple taps of the pedal. The flow gauge was reading 10L/min for 5 seconds for each of these tests