Why does my moka pot sputter? 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.
Folks - please be aware that there are at least two different thicknesses (not talking diameter) for Moka pots. I know that Bialetti makes gaskets in at least 2 different thicknesses.  If your Moka pot design needs a thicker one and the replacement is the thinner one, the seal will never be what it needs to be and the flow of coffee will be affected. Hope this helps.