I have been using a P64 with Mizen burrs (preinstalled from Option-O) and having a difficult time dialing it in for espresso. I'm primarily drinking lighter roasted coffees. I will RDT, grind, WDT, tamp for all of my shots. I have been pulling them on a Bianca so I can see the pressure at the group to help diagnose what is happening. The shot will start out looking good for the first 3-5 seconds and then the pressure will immediately drop below 7 bars even with the flow control most of the way open. I can see the liquid flowing super fast from the portafilter as if a huge channel just opened up. I'm dosing 18g in and looking for around 36 out. Is anyone else having a similar experience with the Mizen burrs?

How does it taste?

Time and flow aren't necessarily an issue. Faster extracting burrs will (often) speed up and blond faster as they pull more solubles out early in the shot. Below 7 bar isn't necessarily an issue (IMO it's likely significantly better than >7 bar for light roasts on burrs like these).

So, what's the issue in the cup? Also a video might help if you're still concerned.

In the cup the taste is fairly weak and bland. The video below was 18 in 36 out in about 22 seconds. This is ground about 2 ticks above burr touch at speed 5 on the p64. On the Bianca I set the flow control to 3/4 and didn't touch it for this shot. I've also included a picture of the bag/beans. They were roasted on March 4th.

I'm surprised that weak and bland is the complaint, I'm trying to parse that in the context of a 1:2 shot, so still thinking a little here.

Maybe a medium light roast?

Do you have another grinder that you've compared the coffee on?

Roughly how many lbs of beans do you think have gone through the Mizen burrs in your P64?

The main thing I noticed from your video is a front (of the machine, left in the video) bias to the flow indicating some issues in the puck prep. Make sure your WDT is even and that any tapping isn't shifting grinds backward.

It's definitely a lighter roast. I don't have another grinder to try it on, but I have had about 4-5 pounds of coffee through the p64. I have had other coffees on the p64 where the pressure goes to 9 bars and then quickly drops and I have to open the flow control 3/4 of the way to keep the pressure above 6 bars. The floor the table is on that the espresso machine sits on does have a slight slant to it so that could cause some of the front bias but I will keep an eye on the puck prep.

OK, sounds good. If you're finding it bland I'd try lowering the ratio to see if you like it with more concentration. If it's a lighter roast it's likely to speed up as you see and is prone to localized channeling at higher pressures. If it was me I'd probably go wide open with 6-7 bar max (OVP) but that requires changing the OVP. Fundamentally I think you'll just have to systematically explore different style shots with the coffee to see what you like most, playing with ratio and flow control valve resistance. Don't expect a lot of body from the coffee, but you should get *flavor*. If not, maybe the coffee just sucks! See what your next coffees are like...

Judging from the video it's channeling top left which indicates puck prep issues, and generally flowing pretty fast. If that is an EPHQ basket, I would switch to a precision basket like an straight walled IMS or VST which will allow you to grind finer. My 2c if it's worth anything.

I agree with this. Mizen's are after conical one of the easiest burrs to dial in I tried in my life, and I tried a lot. Of course if it is not the coffee. Having said that I would not go with Mizen if I am drinking predominately light roasts but the is another thing ...

Light or medium light at 1:2 would generally taste sour. I don't get in the 1:2 range until a solid medium or medium/dark. Of course a lot of that depends on the style of shot too. So it is weak and flavorless but balanced? Hard to say what to do without a bitter/sour assessment.

Give 6 a try for the speed, that is the recommendation from Option O. In theory, this should introduce more fines and help stabilize the puck. I wouldn't open up the flow to maintain pressure, I'd work on puck prep and grinding finer.