detuning a trimmer with a brushcutter is noted in the stihl and echo
factory service info. i was also told this at echo and stihl factory
school. the brushcutter blade has no resistance to air so overreving is
a problem if you hold it wide open with no load. so opening the high
speed screw will slow it down to safe rpm. ...... now when tuning a
carb with the string trimmer head,be sure the string is out to max
cutting length.(no comment on computer users) lucas

http://www.minibite.com/america/malone.htm