I've had flow problems with some of my modern Conway Stewart pens, even after I've corrected any nib problems. Sometime this is due to the ink in question not liking the very smooth Schmidt converters that CS supply with their pens. Having talked to Martin at TWD we came up with the fact that the Faber Castell converters are made of a material that pretty-much matches that of a lot of cartridges and this seems to reduce the surface tension which of course messes up the flow.
Similar Schmidt converters in my Onotos don't cause nearly as many problems, if this is of any use.
An interesting observation, except for the fact that Schmidt manufacture the converters for the current Faber Castells - the only modification being the name that is screen-printed on the metal ring. I have examples of both current models in front of me here and I can assure you they are identical in every detail. Precisely the same converter is used in the Visconti Rembrandt/van Gogh/Salvador Dali/Michelangelo; ST Dupont Defi; OMAS MiLord etc. Most use the K2 design with ST Dupont and OMAS preferring the K5 design with a metal ring also at the nipple end. In some cases, the end of the barrel carries an external thread to give a more positive location in the nib unit. (K6)
The main factor that may affect converter performance is whether the converter has been flushed out, a process I always try to do before using the pen. A cartridge does not of course require a lubricant internally and this may give lower surface tension. A well lubricated (OVER lubricated) converter may occasionally cause the ink to "Hang up" and require a nudge to get the ink to return to the working end. With a piston filler, the larger diameter of the barrel overcomes the surface tension every time.
A properly clean converter will not affect the flow to the nib, any more than piston fillers or cartridges do, because they will always feed more ink than the feed can use. It is the feed that controls the ink to the nib, not the filling system.
Edited by UK Mike, 05 September 2013 - 10:42.