There are different types of felt clunks, ones with a plastic body and the heavier metal ones.
My Flash came supplied with two of the heavier felt clunks. One came with the wing tank and the other was meant to be used in the hopper tank. I used a hopper tank with a pleated paper filter so I used the extra felt clunk in the main tank which is flat and otherwise impossible to get most of the fuel out.

I think that even these heavy felt clunks are way too restrictive so I drilled them out considerably to do away with any cavitation. The opening is now larger than the tubing attached to it. There is absolutely no air in my hopper tank at the end of a flight. Still there is no way I would fly without a hopper or UAT tank.