I have the answer!
Like many others, the gasket (O-ring washer thing) on my trusty moka pot was starting to disintegrate after years of stove-top brews. I carefully checked the size I needed and ordered a pack off Amazon. After popping a new gasket in I got the sputtering, mostly steam, small fraction of a brew that so many others describe. Same scenario of misbehavior detailed here and elsewhere. The problem is described everywhere with irritable, coffee-deprived fervor.
Potential culprits included:
- wrong gasket size
- bent basket (that holds the grounds)
- coffee packed in too tight
- faulty safety valve
- clogs in filter or funnel
- heat too high
- and others I'm forgetting
While it's possible any of those issues can still be a problem, I found the solution to be far simpler: screw the moka pot together more tightly. Literally, when screwing together the top half and bottom half (at the gasket seal), close it more tightly while the gasket is new. I had closed it tightly but brew after brew was getting the same problem but then I tried closing it as tightly as I could and voila!
The thing is, when the gasket is new, the rubber is overly firm from the factory. Even if you think you screwed the moka pot together tightly, the new gasket is not molding to the hairline space between the gasket and moka pot. You gotta screw it together tighter in the beginning while the gasket is still new. As the heat and repeated brews loosen up the rubber the seal will become more forgiving and you wont need to tighten as much. But in the beginning you gotta break in the rubber. Only downside is it can be difficult to take apart.
tl;dr- screw moka pot together more tightly while gasket is new. The rubber is too stiff. Screw it tight and then screw it some more. In time the rubber will be more forgiving.