I got a moka pot a couple of months ago and have more or less settled on a formula I like, but one thing still really puzzles me. The quantity that ends up in the top chamber has started varying considerably. The pot is supposed to make enough for about two full cups, but sometimes only about half a cup's worth will come out and then it'll make the 'boiling over' sound that usually tells me it's done. If I try to keep it on the heat, it'll just keep furiously boiling and no more coffee will come into the top chamber. When I get a low yield, it'll always be really strong, so I can dilute it afterwards and the result is roughly the same, but it's still annoying.
I'm finding it very hard to pin down the variables that make it do this. Of course I always fill the bottom chamber the same amount - just below the valve. I have been varying the grind but I will get two wildly different results even between two brews using the exact same grind setting. I have now settled on a medium fine grind. I have switched from cold water to 90°C - but again, when I was trying both of those options out I was getting varying yields for each. The only thing that I don't keep all that precise control over is quantity of beans (I roughly fill to a finger over a certain marker in my grinder) but I don't see how that would influence things that much.
I must admit I don't understand very well the whole percolation mechanism (in my naive brain, it shouldn't work at all!) so I find it hard to reason through it. Any tips?
PS. If it makes any difference, the model I'm using is a Stellar SC63 6 cup