Stihl Trimmer FS-90 revs hard and spins when idling

[DiyGuy]

When I run my Stihl FS-90, the trimmer head spins and it is revving pretty hard while idling. This seems to have started happening after I did a tune-up (spark plug, fuel filter, air filter). Not sure what could have made it happen, I thought maybe I accidentally turned the carb screw.
I have tried adjusting the idle screw, and turning it does at affect the idles RPMs at all. Kind of stumped right now.

https://imgur.com/4iXQZXG

heres a video of it running

 

[mikehouse]

That's weird.I have tried to adjust my FS70r ala "Steves Small Engine Saloon".I can't tell a difference by ear,so i turn both screws in til seated and then 1 1/4 turn out each.Seems to run strong but no idle problems though.Maybe check him out on YouTube.Can't hurt.

 

[StarTech]

First make sure you adjust the valve clearances to .004".

Then make there a little slack in throttle cable at the carburetor. The idle speed adjustment is the LA screw. You do need a tach as you are looking for 2800 rpm idle and the clutch engages at 3700 rpm; unless the clutch spring is broken.

IF the limiter caps have been removed, the basic settings after lightly seating the H and L are

– Open the high speed screw H
3 1/2 turns counterclockwise.

– Open the low speed screw L
1 1/4 turns counterclockwise.