How to reduce sourness in my moka-pot brew? so most things I read say the common problem with a moka pot is a bitter, over-extracted brew, so I'm confused as to why I'm getting the opposite.

Here are the beans I'm using- http://www.coffeehunter.com/green_coffees/archive/costa_rica/finca_de_alejo_castro, so the decription implies some acidic/fermented tones, but my brew seems to be coming out quite unpleasant.

My moka pot is a Bialetti 1-cup.

My process:

Beans freshly ground with hand grinder (fine grind, comparison with google image results for 'moka pot grind' seem pretty close).

Hot water in the chamber (up to bottom of valve).

Lowest heat.

Results

Getting a pretty fast flow, and no oily bubbles at the beginning of the process. Probably 5-10 seconds between start and end of the flow.

Very sour coffee.

I've tried adjusting my grind, and even when the grind is fine enough to produce quite a bit of sediment, there's no noticeable change in flavour.
What I immediately notice is that you say the entire brew process takes only 5-10 seconds. Even for a 1-pot moka, this seems way too fast.

Vary some other parameters:


Start with cold water in the bottom chamber. While not the best way to brew with Moka in general, it might slow down the brewing process. You should want your moka brew to take closer to 30-60 seconds (my preference, for example). Longer time will allow the sourness to be offset by bitterness (compounds which are extracted after water has been in contact with grounds for a longer time).
Try brewing the coffee as pour-over. This will give you a more neutral preparation that will allow you to determine if the coffee itself is of good quality and too your liking.