Updated list with apologies to @MWJB for not incorporating your comments at this point. Just not quite sure where to put them in without confusing newbies - I’ll give that more thought.
Please review the following and point out my inevitable errors and/or add more ideas. Once done I’ll produce it in PDF form and repost it here.
DRAFT
ESPRESSO POURING CHECKLIST
Foundation checks (before you proceed further)
• Check roast date first. If the beans are more than 30 days post roast they are unlikely to be producing the best espresso possible, other than light roasts which may last up to 60 days post-roast.
• Check pour rate to confirm benchmark grind size is dialled in on your grinder. Get the ratio of bean in to bean out at approximately 1:2 over 25 – 35 seconds.
Prerequisite Equipment (without these, diagnosis is harder and so is consistency)
• Use a naked/bottomless basket otherwise it’s harder to tell what is happening with your pour.
• Weight the beans that are going in and weight the espresso coming out.
• A quality grinder. Hand grinders can be fine. (how to tell?).
• A scale for measuring beans ground and espresso poured.
Then, if …
Too acidic or sour then try:
a. decrease grind size (grind finer)
b. increase brew temperate
c. increase ratio of grams in : out
d. ???
Too bitter then try:
a. increase grind size (grinder coarser)
b. reduce brew temperature
c. decrease ratio
d. ???
Too thin or gassy then try:
a. leave the beans to degas for longer
b. Could be poorly roasted or
???
Channeling or spurting then try:
a. Check your puck prep technique (WDT)
b. Check your tamping technique (even)
c. Decrease brew temperature
d. Check clearance at top of puck (five cent coin test)
e. Increase grind size
f. Dry, warm/hot basket
g. Don’t tap the sides after WDT/tamping
h. Careful not to bump the portafilter when inserting into the group head
i. Are you using a quality grinder with uniform coffee grounds
i. ???
Uneven pour (dark on one side, light on another) then try:
a. Check your tamping technique
b. Check the your espresso machine is sitting level
c. ???
Sloppy watery puck should not worry you, you are not making pucks, you are making espresso. But if it worries you:
a. Check basket size
b. Five cent test (See above)
c. Backflush to ensure release valves are clean
d. ???
Coffee tastes less flavorsome
a. Check the roast date (beans are going stale?)
b. Is the poor blonding (stop the pour earlier)
c. Increase extraction (grind finer, increase ratio of grams in : out, lower dose, higher flow rate)
“Ikky” flavor (often caused by old coffee grinds in the grinder or residual/rancid espresso in the machine)
a. Clean group head
b. Clean shower screen (remove)
c. Clean grinder
d. Clean puck mesh if using one (they retain coffee and get rancid quickly)
e. Clean basket
f. Clean the water reservoir
g. Check the water quality
Other notes
a. Best practice is to change one thing at a time so that you can eliminate some items if nothing changes. If you change two things at a time and it fixes your problem, you won’t know which variable it was that made the difference.
b. Most of us are chasing consistency. Once you change something and you get a great pour, your problem is not solved until you can replicate that result time after time.
c. Keeping a log of what you are doing can help a lot and minimizes the chances of forgetting key information (e.g. optimal grind size for certain roast levels) or jumping to the wrong conclusion.
d. Adjusting dosage and grind size can produce similar results. For example if I’m close to the lower end of my grinder settings and I’m slightly under extracted at 18g, I’ll just dose 18.5g rather than adjust the grinder so it leaves some grinder headroom if the bean changes at all over time. This also may be an easier alternative for grinders that are sensitive to small adjustments and reduces purge waste from adjusting the grind.