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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.
That's what it sounds like

Maybe you turned the wrong screw? Some have high and low mixture screws.

Some cheap trimmers like Echo and Stihl, the grip can twist there by shortening the throttle cable just a bit. That results in a fast idle.

Something is making the throttle cable get shortened, gently pulling to open the throttle plate. Set the idle down to where the head doesn't turn.

Does yours have a fast idle push in button to assist starting? If so, see if that is pushed/locked in.

slomo

There are 3 screws on my trimmer the bottom is supposed to adjust idle speed, but nothing happens regardless of how much I turn it. The top two screws function as they should. I followed Steve's Small Engines guide for resetting and tuning the carb. No matter what the bottom idle screw does nothing, but the top two react.
Here's a picture of the throttle setup on my trimmer. It definitely feels like there is something that is causing the throttle plate to not close fully, maybe related to the starting buttons.
I was planning to try running it without the air filter (safe?) shortly to see if the carb plate is closing.

Remove that plastic trigger group. Take the throttle cable out and shoot PB Blaster penetrating oil inside the cable housing. Cable might be sticking in a shortened position? Remove air filter and exercise the throttle trigger. You should see a closed then open throttle.

slomo

If is not the cable then you will need a tach to do the following carburetor adjustment after the limiter caps are removed.

First lightly seat both mixture screws.
– Open the high speed screw H
3 1/2 turns counterclockwise.
– Open the low speed screw L
1 1/4 turns counterclockwise.

Start the engine and warm it up at varying speeds for
1 minute – if necessary, turn the idle speed
screw (LA) slowly clockwise until the engine runs
smoothly – the attachment must not run.

With the engine idling, turn the low speed screw
(L) counterclockwise or clockwise until maximum
engine speed is obtained.
The maximum engine speed should be between 3,100
and 3,300 rpm.

Use the idle speed screw (LA) to set the engine
idle speed to 3,200 rpm.

With the engine idling, turn the low speed screw (L)
counterclockwise and set the engine speed to
2,800 rpm.

I'll take a look at the internals of the cable tommorow as well as examine the carb plate without the air filter.
When running the weedeater, it did seem like the metal piece that attaches on the throttle cable on the carb end moved up and down as it should.
I am also wondering what that torx screw next to the throttle cable does (in the picture) I think I tinkered with it a little before. Maybe it threw something off?

That is the throttle cable slack adjustment. If over done the throttle will not return complete to idle position due to no cable slack.

Ok I think we really might be onto something with that screw. How would I adjust it to have some slack?

I would say get a small flat blade screw driver that fits that torx head screw. Turn it one way then the other. StarTech will be back soon to "properly" answer your question.

slomo

Great, I already have the air filter off and plan to take a video of the carb with the engine running soon.

** Updates from trouble shooting today:
- With engine off and air filter removed, it looks like the throttle plate opens and closes properly, and the lever attached to the throttle cable has full range of motion
- I tried to run the engine with the whole air filter unit removed so I could examine the throttle plate, but it wouldn't run for more than a few seconds without the housing
- With the weedeater fully assembled and engine on, i tried turning the throttle slack adjustment screw in and out. If it's too tight, the engine essentially runs 100% without pressing throttle. Loosening it up would lower it a little, but it still ran like it did in the original video.
- Could I potentially have a clutch issue or air leak ?

Sounds good sir.

slomo