So, still learning – and hopefully continuing to do so for a lifetime.
I found out why there wasn’t any compression. The piston rings were simply tapered, meaning that I should have paid more attention when mounting them. See an example here of tapered piston rings and how to mount them.
Apparently I had mounted them with the widest side facing upwards instead of downwards, causing the rings to stay flush with the piston, and not expanding outwards against the cylinder wall.
I did however manage to remove them with a little screwdriver, and reinstall them correctly.
I couldn’t however see that these small rings were tapered, no matter how hard or how long I looked at them. They seemed completely identical on either side. I was only able to feel the difference. When flipping them, I noticed that they seemed to have more wiggle room, and wasn’t quite as tight a fit as before.
I put the cylinder and the engine head back on, and kicked the engine over. It fired right up the first time, and there was a good compression. I had the choke on, but removed it quickly, since it was idling with very high RPM’s. As soon as I did that, the machine died on me instantly.
It might indicate that the mixture isn’t rich enough, and that it needs more fuel. I have however not changed anything, pilot sizes etc., so it ought to be running just fine, as it was before.
Fuel is running from the tank to the carburetor as it should, and the carburetor doesn’t seem to be clogged in any way.
I’m out of ideas right now, so feel free to share any advice you might have, would be greatly appreciated.