The nib and feed of the fountain pen has been mysterious to me since the beginning. I know what they do, but I have no understanding of their mechanism.
A few days ago I decided that I really must learn. So I read and watch some YouTube video. The feed main job is to control the flow of ink. Basically, the nib has ink channel and air channel. The ink chanel goes from the ink reservoir to the nib, and the air channel goes from the breathing hole back to the reservoir. The fin stack in the nib is just to soak up excessive flow.
The nib basically just take the ink from the feed and feed it to the actual writing tip.
This knowledge help me immensely, as I have one pen that write weirdly dry. It would start normally, but after a few words it would start to get dryer and dryer. It was really confusing to me, and I turned the nib a lot to increase the flow with no avail.
Then after I cleaned the pen and look at the feed, I found the problem. There was a residue plastic from the manufacturing process blocking the ink channel. So when the feed was saturated (e.g. after long time of disuse), it writes with no problem, but the feed can’t keep up with the required flow rate.