Nick is right on both counts.
Stopping this leak by tightening is not easy. I think you have to do a complete disassembly. Either that or raise the 4 screws, remove heater and temp probe and rotate the whole aluminum block. But it needs to be well over 180C when you do this.
Or you can take it apart and put teflon tape in all the threads - the problem isn't with the nozzle threads - it's higher up in the threaded brass tube.
Another way to stop this leak is by printing with some abs for a few minutes and then switching back to pla. The ABS will leak out of the cracks, and then cook and boil and blacken and gum up and seal all those leaks. UM used to ship a meter of ABS with every machine and the purpose was to seal up the leaks.
But be careful if you tighten with a wrench - brass is not very strong.
Your other problem I agree is underextrusion. Try printing a little slower maybe. Or maybe tighten up the spring on your feeder. Or raise the temperature a little bit - maybe to 240C?
Something is assembled wrong, and your prints look bad because you're underextruding. Make sure you're doing the tightening of the hotend while the entire assembly is hot - I crank it up to 250ish when getting everything snug. Make thermal expansion your friend.
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