I had bought replacement .08 line for use with a precious trimmer and had much left over once it was time for a replacement. Even though the instruction manual on my Black & Decker 40V Max Lithium trimmer said to use only .065, I decided to try using the .08 line when I ran out of line. It seemed to work OK at first but it now seems to feed way too much to the point it is not usable and very frustrating. Even at first it fed a little too much sometimes and I to re-adjust but it was still usable just a bit frustrating. It was purchased in 2014. The owners manual warns against using line other than .065 but it does not say why. Will purchasing .065 replacement line fix my problem?
Using a heavier line will probably not solve the issue. The reason being that thicker line tends to be a bit stiffer than thinner in general, so the head has to be built to suit that parameter. You may just get lucky and the thicker line offsets another issue in the head, so I guess it is worth a try if it costs nothing but time.
I have a B&D 20V and the head auto feed has never worked properly even with the correct line - it always refuses to feed so the line wears down to a stump and has to be manually adjusted. I have not made a fuss about this, I see from online comments that it is a frequent problem and you just have to get lucky that your plastic parts just happen to work right. In my case the battery lasts about 15 minutes which is perfect for the job I have in mind, and the head needs cleaning at the end of the session anyway so the line extension gets adjusted at that time. Adjustment takes less than a minute so it is not a big deal. I'm still working with the line supplied in the head when the unit was purchased 3 years ago so it is not a problem, but it would be if the workload was any greater. What I understand is a greater issue (although much less frequent) is when the head feeds line continuously while in operation and runs through line expensively fast.
Fact is that these simple heads are made with plastic parts and are not designed for anything other than occasional use. Tolerances are critical when the head spins really fast - I'm suspecting that the issue of what mechanics call "float" can happen, when things are happening in the head so fast that the cycle of off-on-off-on gets stuck at either off or on and never gets a chance to reset itself.
Maybe there is an opportunity for Swiss watchmakers to enter the industry and make a string trimmer that gets more valuable as time goes by and will keep working even for your great grandchildren.