Slow extraction [Kingrinder K6 / Delonghi Stilosa]
I'm new and have recently started my journey to make fresh coffee, grinding beans for the first time, but so far have been getting poor results with the equipment I have. I've read a lot of information, watched guides and tutorials and gone through a whole bag of coffee experimenting, but failed to get anything reasonable in so many attempts. It's getting expensive and frustrating.
I have replaced the stock portafilter in the stilosa with an unpressurised bottomless portafilter from Amazon. The manufacturer says the basket can hold up to 16g of ground coffee.
I bought some medium roast coffee from a reputable roaster and let it rest for a week before using. I started experimenting with a dose of 16g but have since dropped down to 15g. There is enough room in the basket and it's definitely not overfilled.
Kingrinder suggest a grind setting of 40 clicks on the K6 for espresso. My K6 is fully closed at -6, so if I understand correctly 40 clicks on my grinder would be an indicated setting of 34 on the dial. Is that right?
In any case, when I tried that it choked my machine. It took 20 seconds from starting the pump to getting the first drop in the cup, and well over a minute to pull the complete shot. From what I understand I should be aiming for around 25-30 seconds as a ballpark figure for a 1:2 shot of espresso.
I guess the grind might have been too fine, so I have tried grinding coarser to 50 clicks (44 on the dial) and while the shot is a little faster it still takes far too long, maybe around 45-50 seconds. It was also pretty sour, and I don't see how grinding even coarser will improve the taste. Wouldn't that just make it even more sour?
It seems there is too much resistance in the basket, but I'm not really sure why. I'm tamping with moderate pressure, although I heard you can't tamp too hard? I'm already grinding coarser than expected. I've seen many people with this grinder use around 30 clicks. Another guide I found suggests a range of 0.22 - 0.45 (https://honestcoffeeguide.com/kingrinder-k6-grind-settings/)
Is there anything else that might be causing slow extraction other than grind size?