After having serviced a few of these machines and owned them, let me see if I can answer your questions:
1) You probably need to clean the shower screen thoroughly. Unscrew it gently with a standard screwdriver when the group head has had a chance to cool down. That being said, the water does not always flow from the center, even when it is clean.
2) You probably need to descale your pressurized portafilter (if it has seen a lot of use). Also, you should NOT be tamping with a pressurized portafilter, only filling the basket level to the top before putting it on the machine.
3) You'll never get a dry puck with a pressurized portafilter on a Saeco. Saeco's lack a three-way valve that sucks the excess moisture away from the coffee when you're done brewing, so you'll always have to rinse your spent coffee grounds out.
I don't personally have experience with the Saeco Aroma machines myself, but all espresso machines usually have a dispersion screen. This is meant to distribute the water flow across the entire width of the portafilter evenly.
The flow speed espresso out of the (pressured) portafilter (left and right holes) are uneven. One is faster than the other. Is it related to the pressure of my tamping?
This is probably related to two things.. Distribution of your grounds and the way you are tamping. I find that I get this behavior when I tamp one side a little harder than the other by accident. When you tamp, you should ensure that you tamping straight down on the portafilter basket, not off to the side.
the coffee ground in the portafilter is with water and is not solid as a cake.
You know, I should read up more on these machines before I post..
The Saeco Aroma uses a pressurized portafilter, which is designed to compensate for a cheap grinder. It is absolutely normal to get "soup" in the portafilter after a shot.
You can only get pucks if you are using a non-pressurized portafilter. You must also have a grinder which is capable of grinding fine enough and consistently enough for non-pressurized espresso extraction. Otherwise, your shots will be very runny and weak.
Don't not buy a non-pressurized portafilter for this machine until you are willing to spend the money on a decent grinder. Otherwise, you won't be able to take advantage of the better quality ground coffee that grinder can produce.
Garbage In, Garbage Out, for every step of the process. From Beans to grinder, grounds to machine, coffee to cup.
I should also mention that a non-pressurized PF is a big plus for the Aroma, but you'll never get a dry consistently dry puck because the Saecos have no 3 way valve. Again, let me reiterate that you DO NOT tamp with a pressurized portafilter. A better grinder will only help. Do you know what kind of grinder you have right now? The pressurized portafilter you're using right now should give you fairly consistent coffee across a wide range of grind consistencies.
First, using a pressurized PF, the grinder is not as important as it is with a nonpressurized PF, not by a long shot, as far as getting back pressure goes. That does not mean that it will be great espresso, you can even get something that resembles espresso with drip ground canned coffee, it won't be good but it will work. The Aroma does not have a 3 way valve so you will have to wait a few minutes for the pressure to drop and dissipate before the puck will not be soup, even with a regular PF. It will never be as dry as a machine with a 3 way valve though.
The stream out of the double spouts of the PF, has nothing to do with extraction, most likely, the machine is not level. Not a big deal unless you are making two single shots in one pull, most here will use both shots in the same drink anyway so it is nothing to really worry about. The coffee leaves the PF through the same small hole in the pressurized PF so all you are seeing is more going to one side rather than the other, as I said, likely not level.
Through the dispersion screen without a PF in place, the water should be centered but likely as not, it will not be with an inexpensive machine such as the Aroma. It should be "soft" with no hard streams or jets of water but again, with the pressurized PF, it isn't such a big deal. Once the PF fills with water, the restrictor is what lets the water out and the pressure will be the same through the whole mass of coffee.
When you move to a non pressurized Pf, grind will be VERY important, but as things are right now, you will get better espresso with properly ground coffee but the PF is what is controlling the water flow through the grounds.
Comments have been made above to grind, tamp and descaling etc, these are very important with a non pressurized setup, with a pressurized PF, you basically have a pressurized drip machine or pressurized french press rather than an espresso machine and those factors really have very little to do with what you are seeing.
In real life, my name is Wayne P. Anything I post is personal opinion and is only worth as much as anyone else's personal opinion. YMMV!
Feed the newbs, starve the trolls and above all enjoy what you drink!
Thanks everyone for your comments. I am using a cheap grinder (about $30) from my roommate. He used it for american coffee. I plan to buy a better grinder with about $100~$200 range (any recommendation). Hopefully, I can head toward having a real espresso. Thank you!
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