Should any coffee be extracted during pre-infusion?

I agree with @JJJ's comment. This seems like a tamping problem.

Still, I advise you to check what you've changed recently. Check whether you've changed a setting or beans. For example, if you switched from Java beans to Ethiopian Sidamo, you may experience lighter tastes. Actually, you need to set up yourself for each new set of beans. If this is not the case, please follow.

As far as I understood based on the use of razor, you're trying to tamp manually. Try to keep the tamping even and use a force between 12 and 20 kgf.

If the machine has an automatic tamping mechanism, very probably it has a setting for that. For example, you may see tender/regular/strong coffee settings which are actually setting the tamping force of the automatic mechanism.

As I said in a comment, I'll lay out a few factors that you need to experiment with.

If I put the grind size very fine (a 3 or 4 on the dial) I'll get less during pre-infusion, but even less during the real extraction and also it'll be sour or whatever; you can't taste the flavour of the coffee. This happens both with the single filter and double filter.

So this is the first factor: grind size. You can probably still tweak it a bit and you'll want to do so with the smallest step the machine allows. I don't know how many steps your machine has (my Oracle goes from 0 to 45, I think) and it allows a decent adjustment in grind size. It's not as good as a dedicated high-end espresso grinder, but it helps to to tune in the right direction pretty well.

If you're not extracting enough then you're grinding too fine or tamping too hard. So that's why I commented about tamping which is the second factor. Those two are related, if you grind super find but tamp it only very lightly then you'll still get a full extraction. The trick is to find a balance between the two and that can change from one bean to another. So instead of taming harder on the coarser grind, you can also try a finer grind with a gentler tamp.

Surprisingly the descale light has never come on, so I've never descaled it. I'm trying to keep the grounds even and I use the razor to even out the ground at the end (I blow out excess grounds dug up by the razor).

This is yet another factor. The razor is probably good for reproducibility, assuming it doesn't ruin what remains in the filter. On the other hand, you might benefit from a bit more coffee in the basket, especially when you're using a slightly coarser grind. So this is also a factor to experiment with.

Finally, there's the extraction time. While it's often advised to have about 30 seconds of extraction time, it's certainly not set in stone. Too long and you may get a bitter cup and too short probably won't get enough volume in the cup (or not enough pressure) but it's something to try.

For example, I've had ristrettos taking 40 to 45 seconds on a 1:1 ratio with a fine grind that were nice and sweet. Indeed, had I stopped those shots at 20 to 25 seconds then they would've been sour similar to your description of the very fine grind.

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I would use a finer setting for the grounds, but my pressure after pre-inf is already very high. Like how is water flowing through the portafilter equally fast with and without pressure? I don't understand the physics of it.

In reply to this comment, it depends on how your machine does pre-infusion. For example, the Oracle builds pressure normally but it has a relieve valve that you can set at a percentage of the normal / maximum pressure. The minimum for my setting is 55%, so it certainly builds some pressure during pre-infusion.

As for the flow rate, it probably isn't flowing as much at the lower pressure. You could obviously compare the time and even compare with an empty basket, that'll flow a lot faster (at least on my machine it does).

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I've had the machine for 6 months. Surprisingly the descale light has never come on, so I've never descaled it.

It's probably a good idea to descale regularly anyway. It depends on the hardness of your water, and it's probably not the culprit here, but I'd try to make it a habit to do at regular intervals (monthly, or bimonthly depending on water quality).

I think this can be due to a faulty machine as well. I've had my machine for 3 years and it has just started extracting during pre-infusion. It does it even with the double walled filters and pre-ground as well so its not the coffee nor tamp thats causing it. I think internally its releasing too much water during pre-infusion now so something has let go inside the machine. If i take off the porta filter and just watch the pre-infusion phase is pushing through a lot more water then it used to. Its not leaking either so its not group head seal. Might be time to upgrade to an Italian machine i think.