I have two HP printers, deskjet 3420 which uses HP27 & HP28 cartridges. I
have successfully refilled these cartridges over several years.

My newer deskjet 3920 uses HP21 & HP22 cartridges and I have been told in
the past that the only difference with these and the other 27 & 28
cartridges is the amount of ink they can hold 5ml as opposed to 8ml.
However I have been unable to successfully refill these cartridges.

Firstly when I insert the ink in the HP21 & HP22 it feels as though the
syringe is going into "foam".Whereas when I refill the HP27 & HP28 its the
normal "sponge".

I have tried several HP21 & HP22 , refilling when half full, empty etc no
success with any of them by this I mean the ink goes in the cartridge
alright but print output is bad.I have tried cleaning cycle religning etc.

Can someone say if they have had success refilling these cartridges ( and if
so what's the secret) because I'm seriously thinking of chucking out the
printer.

Martin