Trying to refine my technique and seem to be steadily improving with BDB and Sette 270. Overall, I am pretty happy with results, but always room to improve.
One thing I have noticed is that my second shot consistently runs faster (6-8 seconds on average).
I feel like I have everything pretty consistent - 18 grams in, 36 out, use distribution tool and leveling tamper.
Preinfusion set at 15 seconds, first shot running around 42-44 seconds or so.
The only thing I can figure out is that the portafilter is fully warmed up by the second shot, which maybe accounts for the difference in shot time?
Generally, my warm-up routine has been once the BDB is up to temperature, I would pull the first shot with no pre-flush. I know, not ideal but I didn't previously appreciate that step. So definitely the portafilter was barely warm for first shot, and generally hot for second one.

Now that I am getting more picky, and want more shot to shot consistency, I want to correct this as I can tell the difference in taste between shots.
So tonight I let the BDB get up to temp, then ran a blank double shot. Then I did my first shot, followed by a second one 10 minutes later. I feel like that helped, but still was 4-5 seconds faster on second shot, and I also still feel the portafilter was even hotter by the time I did the second shot. So maybe I could have let it warm up longer before first shot?

Could portafilter temperature explain the shot time variation? If so, what is your BDB warm up routine?

Appreciate the advice!

Try leaving the portafilter in the group even when not in use: on power up, it will be heated with the group. You shouldn't need to pre-flush at all with the BDB.

When I power on the BDB, I do have the portafilter in the group. The BDB heats up so quickly, though, that portafilter is barely warm by the time it can pull a shot. Are you letting it warm up longer?

Shots run slower when the water is hotter, not faster. How are you grinding the two shots?

Iowa_Boy wrote:When I power on the BDB, I do have the portafilter in the group. The BDB heats up so quickly, though, that portafilter is barely warm by the time it can pull a shot. Are you letting it warm up longer?

Sometimes longer, but my portafilter is very hot to the touch about the same time as the boiler is ready. I'm using the bottomless, though, so less thermal mass to heat up. The group head is electrically heated, so I'm surprised that yours isn't that hot (unless it is the spouted, maybe). Perhaps give it 3-5 minutes longer and see?

IME, typically, the principal culprit when things run fast or slow is the bean/grind.

To eliminate (or confirm) the machine as the cause, pull a couple of blank shots through one of the pressurized portafilter baskets, or make a couple of 30s pulls into a blind basket before commencing your usual routine and see if the problem remains.

Same with the grinder... Grind out the first dose into a cup and set it aside. Now commence your usual two shot routine and see if the problem remains.

Report back here.

-Peter

Thank you everyone for the input! Here is the results of tonight's testing.
First thing I did was clean the Sette 270. Haven't done that before, and it's 2-3 months old.
For a low retention grinder, there were a lot more old ground coffee inside than I expected.
Next, I let the BDB warm up an extra 10 minutes or so. I did some blank pulls into blind basket as Peter suggested.
Then 18 grams into Sette, and time for first shot.
First shot was 36 grams out in 36 seconds - which was the faster second shot time from yesterday. Interestingly, it had basically had no bitterness that my first shots usually have. I never thought that could be stale coffee from the grinder causing that taste, but perhaps it was. In any case, onto shot 2.
Between shots, I briefly rinsed and dried the basket. Also, briefly turned on BDB to rinse screen, and then dried screen with microfiber cloth. I haven't done this previously.
Shot 2 was 18 grams in, 36 out in 42 seconds. Huh? That wasn't what I was expecting, although it was my goal time. Never had shot 2 take longer than shot 1. Taste was outstanding (Paradise roaster Espresso Nuevo).
Decided to do a third shot for scientific purposes.
Rinsed and dried basket again, then rinsed/wiped BDB screen.
Shot 3 was 18 grams in, 36 out in exactly 42 seconds again. Didn't taste as 2 double shots at night is already pushing it.

Not sure what to conclude from this.
For tomorrow, I will definitely run the Sette a bit before first shot to get any stale residual out. At a minimum, I feel taste is better.
Will continue to rinse basket and BDB screen between shots, it's effortless and takes very little time.
Is the time variation I am seeing maybe just a factor of the Sette not being exactly consistent? Am I expecting results to be too repeatable?
Doesn't seem like warm up temp is really playing a role here.

I do rinse and wipe the PF/basket and flush the group after every shot (but not thru the PF), usually also use a laboratory wash bottle to rinse the group and screen as well, since on any given day, any given shot might be the last session of the day. So that might be a factor at work, here, too.

I'd be surprised if the Sette has that much retention, but I've been surprised before. I don't use a hopper grinder anymore, and single dose, so I didn't even think of it being the Sette. Glad you got it solved!

After cleaning the grinder, the next time you grind it will start packing the grinds into the clean spaces, so your first grind may have been a smaller dose then the next ones.

That would make sense. Will report tonight's results!