I know this is a late reply, but I think you've got it right.

If I understand the function of the sensor feed head correctly, it feeds once the tension on the line reduces, which is a result of the line growing short. If you use too heavy a line, it will never feed, or will only feed a bit. If you use too light a line, it will continuously feed and empty the spool quickly.

I tried using the new spiral line, and because it is actually lighter, it used up a nearly full spool in minutes. I replaced the line with the standard .080 round stuff, and it is now behaving.

I'm considering replacing the head with something more modern, like the bump feed type.

Actually. it was very timely reply as I was about put this machine out for a "curb alert donation " . I will see if I can scrounge some samples of different sized line and try them first .

I see the consumer gas trimmers that need minor fuel system repairs for free (or close ) and I will just get another with better feed mechanism .

I landscaper friend lent me a more powerful Stihl trimmer with an aluminum Oregon head with manually installed 0.155" line like the the "trimmer mowers use" . That is Overkill for most people . That worked well for edging and cutting some small brush , If my trimmer had more power and was used a lot , I might consider buying one of those heads .

- Thanks