slipping strimmer ?

It sounds as though the centrifugal clutch is slipping.
This could indicate a problem with the clutch requiring repair. OR.
The other thing that could be happening is that during cutting the blades get covered in sap (esp from spruce/firs) which goes sticky and starts to bind them causing the clutch to slip.
Give the blades a good clean before you start cutting & give them a spray with WD40 or similar. Then during cutting apply a little more spray now and again to stop sap from clogging blades. Clean blades at end of use.
I hope this helps.

Robin S
August 2008