I have an old Sthil FS80, heavy duty trimmer/brush cutter. It is about 40 years old but was completely rebuilt by Sthil about 15 years ago. It has served me well but has received no more than what I would call moderate use. I hadn't used it in several years but it worked perfectly last time. I did remember to drain the fuel when I put it away. Well.....recently it was needed for a Honey Do project. I was not able to get it started. Used fresh fuel and carefully measure Sthil oil. Nuttin'
Tried many times to start, even using ether in intake. Still nothing. The spark plug looked OK but, just for the 'H' of it, I bought a new one from the local Sthil dealer. About $3.00.
Put it in and the trimmer roared to life on the first pull.
Not going to guess why it worked last time but all I know is it works now and I saved an expensive bench charge at the Sthil fix-it shop. Color me happy.