I have an old 6-cup moka pot (bialetti) that I recently replaced the filter and the rubber gasket (bialetti brand), however the extraction time seems a bit long, bubbles to soon(almost immediately) and sputters out- resulting in that bitter over-extracted taste. Strangely, it did work fine just a few times after I replaced the gasket but soon after the sputtering started. Thinking that maybe the rubber gasket was faulty, I replaced it again, but still the same problem. In the past, this moka pot has produced good brews with a nice uniform flow.
Could it be something is off with the safety valve i.e. not sealing enough and allowing pressure to escape?
By the way, I also have a 3-cup moka pot one that I have tested with the same process (pre-heating water, grind level, grind amount, medium heat) and the results are consistently good.