Would love some help here - Excited newbie to espresso and after some early success with a dark roast blend, I'm having a really difficult time getting shots from a light roast to not channel significantly (at least I think that's what's happening).

I've attached photos - my prep (WDT, tamp, etc), as well as the shot itself. (Have videos of the process as well)

Using a Flair 58 & Niche grinder, as well as Flair's low flow 18g basket that allows for dose between 16-22g.

I'm following the same parameters the roaster instructed, 18.5 in, 40g out in 23-27 sec. My shots look visually similar (substantial channeling) with a variety of grind settings. Curiously, I'm still hitting my target times with my setup, and when working with a different coffee (darker roasted blend) the same prep steps are working well, and the shots from the dark roast seem to flow really evenly - so, I am confused about what I should adjust.

Could I be messing up my WDT/tamping? Should I adjust grind or dose? Taste isn't terrible actually (despite how the shot looks), but significantly less clarity from the roaster and tastes unevenly extracted (bitter and sharp/sour)

Coffee:
Washed Colombian, lighter roast (not Nordic light)
roasted on 1/24

Recipe (suggested from roaster)
18.5:40g
23-27sec

Prep steps
- Heat Flair 58 on highest setting
- Boil water
- Pre-heat portafilter in kettle, wipe down before adding coffee to make sure it's dry
- Grind 18.5g on Niche - setting 10 gave me the closest timing to roaster's suggestion
- WDT (touching bottom of basket, moving to middle, then to surface), then tap bottom of portafilter against bar
- Tamp , using Norcore V4 calibrated tamper
- Add puck screen, and tamp again lightly

Brew steps
- Fill chamber with boiling water
- Using Flair's low flow 18g basket that allows for dose between 16-22g
- Pre-infuse at 3 bar until first few drops
-Ramp to 7-8 bars for most of shot, drop pressure slightly to 5-6 bars for the last 5-ish grams

The naked portafilter (and taste) seems to be exposing some serious mistake but I am struggling to identify what it is. Appreciate help!

Have you try grinding coarser?

Yes - similar result except water rushes through even faster

Are you getting 40g in 25s from first drop or when you start the pull?

If you lock in and remove, do you see any marks on the cake?

What does the top of a spent puck look like?

This. Do not try to match the roaster's recipe. You don't have the same equipment. In my experience a fine grind and a very long pre-infusion 10+ seconds can help get an even extraction with light roasts. Also, after gently ramping up the pressure raise the lever just a touch to let the puck expand and saturate evenly. This will help reduce channeling. You can even do this action more than once if you need to grind finer to hit a shorter ratio.

Thank you! I'll try this out. Do you suggest grinding finer with these changes as well?

Also, do you happen to know if the results in getting are a result of me doing something incorrectly, or if this is just fairly common for light-roasted coffee in espresso/manual espresso machines?

Time from after the first few drops of PI and when I begin ramping pressure to get to 40g is happening in about 25-27 sec

I think it's not uncommon for light roasts. I've seen threads like this regularly here. I suspect it's due to grinding too fine (too many fines + perhaps uneven distribution that can't be fixed with WDT) and pressure being too high for such puck.

As another poster said above, you might also try grinding just a tad coarser if your 25-27 sec is from the first drop. Try maybe 20-25 sec including 5-10 sec of pre-infusion. It's a fast yet controlled flow shot that gives me consistently good results for most SO light roasts on my Flair 58.

You are getting different advice from two different styles of espresso. You can properly extract with either type.

If you want a longer shot, you may want to grind coarser to help reduce channeling. Large particles will not extract as readily so you need more water to complete the extraction.

If you want a shorter shot 1:2 or lower you can grind finer, but this has higher risk of channeling. My technique for dealing with this is the long preinfusion and also after applying pressure for a few seconds, to raise the lever gently, slightly, to allow the puck to expand and fully saturate. Then again ramp up pressure slowly. The first time you lower the lever with a fine grind the resistance may seem a bit too much, but after the second time you lower the lever the resistance should be significantly reduced and you should get a beautiful flow. This is super easy to do with a lever and you can feel the resistance which can tell you how the puck is doing. If resistance is too high, raise the lever to let the puck expand and lower again.

None of the above is a substitute for following your taste buds. It will take some experimentation and practice. Don't get too frustrated if it takes time to get it right.