Identify a mechanical problem...
First, you need make sure that you're not stuffing from severe channeling. You should be able to see evidence of this in the puck. When you disengage the portafilter, check the top of your puck for holes, like someone poked it with a pencil. If you have these, you have channeling. This could easily be caused by poor dosing technique or a clogged shower screen. Start by popping out your screen/gasket and making sure that everything in clean including the underside of the group. When prepping the PF, try to keep things simple and short, doing just enough to get an even bed of coffee. Do give the PF a gentle downward tap on a hard surface to settle the grounds. Don't tamp them like you're the Hulk. The less you do, the easier it is to repeat consistently.
Moving on to making a espresso...
If you can get over to the UCLA area, I would hit up Espresso Profeta to buy some Vivace Dolce or Vita beans (I can't remember which they'll sell you). While you're there, have a shot and a cap, and ask the baristas what their in/outs are and what their shot time is - however, I think they are sufficiently old school such that they might not be doing things by weight, so get whatever info you can and observe how much espresso is in your cup before giving it a stir. If you like it, great now you have a goal. If you don't like it, take a moment to reflect on what you don't like about it. Espresso is not easy, and most baristas are happy to talk about it if you tell them that you're struggling with it at home.
Next, get a scale and start doing things by weight, grams in / grams out. It takes skill and experience to do things by taste and intuition. If you don't have those, you need measurements to make sure that you're consistent and to know what it is you're actually pulling. If you're using a 20g VST, you should commit to dosing in 20g and trying to adjusting your grind to get the shot you want. That of course means that you need to have an idea of what you want. If you liked the shot at Profeta, you're probably going to want to shoot for close to a 1:1 brew ration, i.e 20g of grounds into the portafilter : 20g of espresso into the cup. If the volume and intensity of a 20g:20g shot seems to you're liking, move forward with finding a shot time that tastes good to you. If you need a place to start, start with a 25sec shot. There should be almost NO acidity in Vivace Dolce or Vita...if you're shot is sour, it's under extracted and you need to make it take longer by grinding finer (still 20g in).
1. Vivace Dolce or Vita Beans
2. 20g in = 20g out in 25sec
3. Adjust: find a brew ratio that's acceptable, then adjust grind to control extraction time
If you can't slow down a shot to hit 20g:30g in 25sec, you have a problem with your technique, your machine, or your beans. If you're using Vivace purchased from Profeta, it's probably not the beans.
russel at anacidicandbitterbeverage dot com