Maybe a drunken stihl engine assembler dropped the crank on the floor? no idea..
If the crank/bearings are o.k. I'd just drop in a new piston/rings and run it for another 2000 hours.
Sure, the air filters are whimpy, but the saws sure seem to last. Most of the dirt gets inside the filter when the filter gets completely blocked - then it pulls in around the edges. I have a few guys that now keep their filters clean and there is far less dirt in the wrong places. The older the saw gets the more it spits back. Most of the black sponges are missing. Once it really spits all over the filter the fine dirt togther with the oil matts the filters really bad. I do like the MS200 style - completely enclosed filter.
As for the Zama L screw.. no idea.. never seen that. Maybe there's a bad vibration from something wrong? The Zama L screw has a tight thread class and no spring, and I don't like that approach, but never have seen it give problems. I'd put locite 222 (or so..) on the outer threads of the neede if it's not tight.
And I wasn't trying to rev you up about your pressure tester (we've been around the block on this before), but 90 seems very odd for a saw with a piston etc like that. The reason it came in was probably the carb screw, but it was likely running poorly just prior to that... and it sure won't run at 90. At 130 they feel very limp in the pull.