Possible channeling - BES920
Basically, to keep a long story short, my shots start off quite good, often a bit on the slow side (dripping), but then end up quite fast. To give an idea of time, pre infusion is left set at the factory 7 sec, and the first drips out the spouts doesn't occur until about 14-16 seconds. even though its quite a big gap, it still looks ok for the first 5-10 seconds and then it will change quite quickly to a rather fast pour and total time is still around 35-40 sec on the shot clock, which is only 20-25 sec from when it started pouring, sometimes less.
I may be worrying for nothing as the coffee still tastes quite reasonable.
what i found when i first bought the 920, was that breville had got rid of the tapered sides in the double basket compared to the 900. due to my pullman tamper being matched to my old bes900, and the new bes920 basket being slightly bigger, i continued to use the double basket out of the 900 until i got around to getting a new tamper base matched to the 920 basket. This problem i mention didn't happen when using the 900 tapered sided basket, and i just assumed it was because the tamper didn't quite fit properly in the new, more vertical sides of the 920 basket.
so, i eventually got around to getting a new tamper base and bought a new pullman double basket to match it to. These look very similar to the 920 basket and i have read that the 920 breville baskets are actually quite well made. Anyway, what i discovered is that while having a tamper matched to the basket did help, it didn't completely solve the problem and i've been living with it since. thought it was time to get some thoughts off the forum.
i'm guessing that the different shaped basket is highlighting some flaws in my technique that the previous tapered sided basket was hiding. doing some research i think it may possibly be to do with distribution? i always fill the basket in 2 steps, knocking the portafilter on a rubber mat after each step to settle the grinds. then tamp, polish, wipe rim and tabs and insert etc. i've read that because these straight sided baskets also tend to have the holes extending a lot closer to the edge, that distribution is quite critical now and could be the cause of what i'm experiencing.
anyone else had similar issues?
i'll try filling the basket in a single step and then levelling off and tamping and see if that goes any different. machine is paired with the 820 smart grinder pro.
Maybe too finely ground and then channeling?
Coarsen your grind and see how it affects the pour.
A common cause of this (maybe not yours but a place to start) is stale/older/supermarket beans being ground too fine to get an extraction.
Starting point for diagnosis is a good machine clean, fresh beans from a reputable source, a 20g dose ground at 8 on the Smart grinder. The pour might be too fast or too slow but it should be even and you should not see much change on the pressure gauge throughout.
I would also also set the pre infusion time to zero while working out what's going on and aim for 28secs.
The 920 is a very forgiving setup and whilst good distribution will help it the machine seems to tolerate a fair degree of clumsy dosing. Don't ask how I know that
Machine is only 9 months old and is like new. I back flush with water after every use. Beans are fresh from a reputable roaster. Pressure sits just over 9 bar and stays over 9 for the entire extraction. I have tried more coarse but it is worse.
I'm not new to these machines. Had a 900 for many years before this one. The only real change is the double basket shape.
The only other thing that might be worth mentioning is that a couple of months ago I ran out of beans so grabbed some local stuff from a local cafe rather than direct from the roaster. Obviously it was stale and I had to move the adjustable top burr down one or two holes from the factory setting, so now with good fresh beans I'm grinding up around 16-17 instead of under 10 previously but I can't really see how this would affect it.
Which is all good. So where do you feel its going wrong?
I've always gone by the actual pour time from when it starts to come out the spout. Now, even when it starts off very slow, it's still all over in about 20sec. The breville shot clock reads about 35-40, but like I said, sometimes pour doesn't start until nearing 15-20sec.
Maybe the beans are a bit to fresh. They were only roasted on 31st. Crema is very thick to start with but did dissipate to about 3-5mm by the time I've done the milk.
Maybe I'm worrying for nothing.
Doesn't sound too far off. Try setting the pre infusion time to zero for a few brews to get a more conventional sense of whats going on. You should see the first drops in 6 to 8 seconds and be finishing up around 28 to 30. If you get that your in the right area and its just the pre infusion altering the numbers.