Second consecutive shot is always way more restricted than the first [Breville Bambino, Turin SD40 equivalent]
Grinder: Turin SD40 badge engineering equivalent (branded Xeoleo, incremental steps and 8 spline burr or whatevever, forgot the terminology)
Machine: Bambino w/ breville singlewall doubleshot basket and stock portafilter
Typical dose: 16.5-17g, ground with a running start.
Anyway, I'm consistently having this issue where my first shot pulls pretty much perfectly after dialing in, but my second shot will often be almost completely choked off. I find I either need to drop my dose by 0.50-0.75g, or increase grind size one step and increase dose by maybe 0.25-0.50g. If I let the machine rest maybe an hour or a bit less, it will produce another first shot according to my initial settings. I always preheat my portafilter by running a doubleshot through it empty, and always rinse my portafilter between shots, then reheat with another empty doubleshot.
I've read things about retained stale coffee, but I don't really think that's it. I know the SD40 is a high rpm conical burr grinder, so I'm wondering if I'm seeing heat expansion of the burrs changing my grind size, or if it's perhaps something to do with the machine characteristics. Anyway, any insight would be appreciated. I don't know if I can "fix" this without upgrading something, but I'd like to know the source of the problem.
Thanks for your help!
P.S. One thing I need to do is measure the output dose of my grinder and check for consistency. I just haven't remembered to. I wouldn't imagine it to vary that much but maybe I'm wrong.