Hey Brian, welcome to the community!
You've got a pretty capable setup on your hands, so give it a little time and practice and you should be well on your way to enjoyable espresso! I highly recommendchecking out Randy Glass' website and particularly his easy guide to better espresso at home
bcparfitt wrote:Most of the time, it's too bitter or sour.
Sour and bitter are typically at opposite ends of the temperature spectrum. Are you doing a quick flush to stabilize your boiler temperatures before every shot? Sour can also come from over extraction due to channeling. Are you using a bottomless portafilter? They are a very helpful tool in diagnosing extraction issues.
bcparfitt wrote:Silvia Temperature set for 207F
Pre-infuse for a second
Pause a second
Shot pulled in 27 seconds
This makes me assume your Silvia is PID controlled. Keeping everything else constant, I would adjust your temperature lower until shots are consistently sour, and then adjust them higher until they are consistently bitter. The sweet spot for a particular roast should be somewhere in the middle.
bcparfitt wrote:Questions: Do the volume and weight seem correct? Anything jumping out at people as glaring problems from what you see so far?
While there is nothing wrong with higher brew ratios (17:42 is roughly 1:2.5, so a bit on the lungo side, but still in normale range), I find that they are more difficult to nail consistently than a ristretto or a normale closer to 1:2. Basically, you are asking more of your grinder and prep with respect to uniformity and distribution to run that much water through the bed without opening up a channel, blonding or highlighting some other error in the process. If the shot was great, then by all means, carry on. But it wasn't, so I think it's fair advice to start out a bit tighter until you are getting something you are consistently pleased with. Personally, I would shoot for something in the 1:1.5 to 1:2 range until you decide you want to chase after more yield in the cup, but above all else, keep whatever brew ratio you decide on constant until you get consistency in the cup. Change only 1 thing at a time...
I find shot time to be the least important of the variables. I tend to pull a 1:1.5 ratio for straight espresso with the goal being a steady pour that completes at the first sign of blonding. I go a little longer for milk based drinks, and with my aggressive preinfusion, I don't worry much about how long the shot takes, I just monitor the pour and adjust grind to ensure I'm still getting a good extraction when I reach my desired yield.
You're in the ballpark with your yield and time, but are probably blonding early, at 20 seconds or less, given your struggle with flavor and higher yield. I would stop the shot at the first sign of blonding and then adjust the grind such that this occurs when you reach your target yield and is in the 30 second range. I'm not certain your pulsing preinfusion is doing you any good, but it probably isn't hurting anything, either... Silvia's vibe pump has a fairly soft ramp up curve, so you could probably eliminate some shot-to-shot variability by just locking on and pulling your shot. At least until you're satisfied with the consistency of your shots.