Need help dialing in Bambino Plus

[hallydamaster]

My wife gave me a Bambino Plus for Christmas.
I really like the functionality so far, but I seem to have a little trouble dialing it in.
I own a 1zpresso J-Max, which I used with another espresso machine that I found at a thrift store (Rommelsbacher 1500). The old machine came with only the pressurised basket, and actually made all right coffee (not real espresso).
The Bambino Plus comes with non pressurised basket, and I actually have a little experience with puck prep from my friend's machine. He has much better gear, so I'm not gonna compare the experience, but I was able to produce a good espresso on that.
The trouble I'm having is that the espresso I produce is quite bitter. I tried making the grind coarser, but I feel like I'm moving out of the espresso range for the J-Max (I'm in the ~160 clicks range.
The shot I produce is 18gr in 37 gr out in ~25 seconds, which seems close to the desired 1:2 yield.
My beans are dark roast, and roasted 21. November.
Are my beans the problem or can I tinker with some other parameters first?

[beans+crumble]

I had good success on my Breville Barista Express when I down dosed the basket just a bit... 16g was a good spot for me and at the time I was using Peet's coffee beans which are pretty dark in roast. So maybe reducing the dose by 1-2g and see how it tastes. Other possible options include doing a more tight ratio (i.e. 1:1.75, 1:1.5) if using a darker roasted bean and/or decreasing the brew temp slightly can help with darker roasted beans. And like you said... puck prep is very important now that you're not using a pressurized basket. It will just take some time to get used to the change. Good luck!

[Jeff]

I don't know how dark the roast is, but darker roasts tend to be more bitter than medium roasts. Are there shiny spots or visible oil on the beans? Are these the same beans you were using on the other machine? Have you tried to make some filter-style coffee with the beans? Were they very bitter if you did?

Darker roasts at a month off roast may be more challenging than fresher roasts. If you find that your shots are flowing too fast, you might want to try a bag that is maybe 10-15 days from roast.

[hallydamaster (original poster)]

@beans+crumble
I dosed down to 16gr and dialed my grinder back to a finer setting. It took a longer time for my shot to run through, and ended up yielding me 36gr in the cup. The shot tasted fantastic, bitterness all gone and found a little sweetness.

@Jeff
I think the beans are medium dark, has some light spots and no oilyness to them.
I would really like to find better beans, but unfortunately living in remote Iceland is giving me a hard time finding fresher beans.
Considering getting fresh unroasted beans, and roasting myself.

[beans+crumble]

That's great to hear! Nothing more frustrating than trying to figure out how to dial in new beans, machine, or grinder when just starting out. Those days bring back lots of frustrating memories! Hope that you continue to get good tasting shots and you enjoy your new machine!