In November 2007, I purchased a new Duofold Centennial from eBay. It has a fine nib. I use Parker bottled ink/cartridges.

The pen produces a fairly wide line – certainly wider than the Sheaffer (regular nib) I had previously.

I sought the advice of a local pen shop, who suggested that I change the cartridge, as the existing one may be defective and letting out too much ink. I did this, but it did not make any difference.

Is it possible that the nib itself may be defective? Has anybody else had a similar problem?

Many thanks.

Have you tried the converter?

The nib could just write fatter than the Sheaffer.

My Duofolds do write on the wide side, but not two sizes wider.

Sorry, yes - my original post should have said I changed the converter rather than the cartridge.

It doesn't seem to leak, although refilling the converters through the pen does create a lot of bubbles, suggesting it lets in too much air. I tend to remove the converter each time and fill it up on its own then put it back in the pen. This can be quite messy I have had not had to do this before. All quite frustrating!

I have never removed a converter to fill it so I can't help you there.
Filling the converter while installed also fills the feed and nib of the pen and primes it for writing.
Just remember to expel a few drops of ink and draw in air to clean the air channels.

Not all fine, medium or broad nibs are created equal. I have a Duofold Centennial with a medium nib and I would say that of all the medium nibs I have, about 36, it lays down the widest line. I have some fine nibs that lay down a line about as wide as my narrowest meidum nibs. The other variable that could be affecting the line width is the wetness of the nib and this can be adjusted to write drier. I do recall reading on FPN that the nibs on Duofold Cetennials generally lay down a wide line relative to other pen makes; unfortunately I can't remember or find the thread discussing this topic.