I had occasion to visit a barista in the city today, on my way from one work stop to another. The double shot of Italian roast (they made sure to let me know that) was not bad, just slightly sour... which I noticed more and more after I had finished, and was walking to my next stop. The aftertaste was unmistakably sour.

I'm now home, and the robusta-less shot I made for myself upon my return is 15 minutes in the past. The shot was neither notably sour nor bitter; it was sweet and potent and delicious. And the aftertaste is not sour and not bitter. It's just pleasant.

I had been wondering about sour. I've now got a baseline, and realize I've never really pulled a sour shot! Except last week with a Guatemalan light roast, and it was not bad. Right around the same sourness as today's barista shot.

Recipes on the ViaVenezia:
Robusta espresso blend: 5 seconds or so per-infusion, 20 seconds gradual increase, 8 seconds full flow.
Non-robusta blend: temp-surf to slightly hotter water. 10 seconds or so pre-infusion, 15 seconds increase to full, 8 seconds full flow.

Note I'm not weighing anything. It's entirely dependent on time.

I'll have another attempt at the Guatemala light roast this weekend. I think I know what needs to happen. Surf to even more heat, not much more though. Perhaps two more seconds on the steam button. Then maybe 12 seconds pre-infusion, 12 seconds increase to full, and 8 seconds full flow.

i think temp has much the same effect. let the shot (or drink) cool and the you get the (more) true flavor.

I agree. Heat masks a multitude of sins in a cup. One of the best I ever had was an Aeropress Americano using 100% Kona that I roasted in my Hotttop. I had consumed some when hot and then we kit the road. It sat in the cup holder of our RV. Remembered about an hour later, I picked it up while heading down the road and it was liquid dessert.

And as the OP stated, paying attention to the 'follow' is important. Much can be revealed as the coffee sits on the palate after the swallow.

Followup: the Guatemalan light recipe worked perfectly. It was delicious, sweet. The aftertaste is rolling around in my mouth right now... maybe not quite a god shot, but close.

The next time I have to reduce the dose a bit. The grinds bloomed like crazy in the basket while pre-infusing! Where there had been a gap of nearly 1/4" from top of grinds to top of basket after tamping, when I removed the portafilter afterward the grinds came all the way to the top of the basket.

Incidentally, I have not changed the setting on my grinder. It's identical to what I'm using for my other beans; I've not changed it in weeks. When the beans age past the point that this setting works, those beans go into the "morning coffee" mix.

This hobby is starting to get interesting.