Espresso that bleeds out in a few seconds will taste weak, bitter, and somehow empty. A fast flow is the result of water not meeting enough opposition as it flows through the coffee. The obvious culprit here is a grind that’s too coarse, with excessive distance between particles. Rotating the grinder to a slightly finer setting will increase the surface area and slow the flow of water, thereby extracting more of the flavor. Only make small changes here, as subtle adjustments to the grind can have profound effects on espresso.
The dose will impact this as well, more so than you might think. If your basket is under-dosed the puck is not dense, so water can flow through very quickly. Experiment with adding a little more coffee so that once the grounds are evenly distributed in the basket, they are just below the top of the basket. Once you insert the portafilter, the grounds should be loose enough that they won’t leave a mark on the machine, but dense enough that they still feel heavy. Of course, if you overfill the basket, it will be the opposite issue and the water won’t flow at all, so aim for somewhere in between rather than trying to load up as much coffee as you can get.
Bad prep ruins a perfectly good setup. When the bed isn’t flat, water will find a path of least resistance and run through the lower part of the bed, which will then speed up the extraction. This will leave you with a seemingly nonsensical combination of too fast of a shot and bitterness. Level the bed by running your finger over it, or by using a distributor after you dose. Tamp straight and with constant pressure. An angle on the tamp will always compress one side of the puck more than the other, and water will run down the softer side.
You might not realize it, but a cold portafilter can also play its part here. It will lower the temperature of the brew water, causing the extraction to run quicker. Warm your machine up properly and do a short flush to equalize the temperature. I have seen many new baristas eager to get started and speed up this process, but temperature is crucial if you want consistent results. If your first shot of the day is fast but the rest are normal, this could be the reason.
To develop your sense of the process, repeat the above exercise many times, not once every week and a half when you happen to think of it. Spend about fifteen minutes pulling two shots in a row, changing only one variable in your process between the two shots. See the difference in the stream from light and watery to thick and syrupy. Drink them while still hot and pay attention to the body as well as the taste. Over several sessions, you will begin to understand the look of the stream as it slows down the shot before you even taste it. It will no longer be a guessing game.