My sthil fs45 weed wacker I had to take out the shaft and now my throttle won’t go back to idle when I rev it what could it be?

SOURCE: weed wacker

check the fuek lines for hole and leaks, check the fuel filter and clean if necessary. remove the carbuerator and inspect the gasket; it should be soft and flexible, if it seems hard or brittle replace it. clean the carb with carb cleaner sray (available at any local auto parts store) and compressed air. make sure you get inside the small fuel rail holes. if that doesn't work have the carb repaired at a service center or replaced.

SOURCE: I have a 32cc crasftzman weed wacker need how to

This can be a little tricky, the filter is in the tank connected to the line. The line is pulled through a hole in the tank which seals only because the line is the correct outter diameter to seal the hole and not collapse the interior. Fuel line and filters are available at most places that carry outdoor power equipment parts but you must make sure the line is the exact size inner and outter as the original.
If the trimmer has a primer bulb the line goes from the tank to the bulb and then to the carburetor, a second line goes from the carburetor back to the tank, don't get them mixed up.

SOURCE: Does Bolen's weed wacker BL100 17" cut 31 cc require a mix of gas and oil. Thank you

I was able to get information from someone's manual. I can't find mine. Would like a manual.

SOURCE: starts then dies stihl fs45/weedeater

I sounds like it sucked up some debris in the bottom of the fuel tank and obstructed fuel flow somewhere. Start by blowing out the in-tank fuel filter (You will have to fish it out with something), then load the line with carb cleaner (wear safety glasses) put fuel filter back in line and crank it a few times to pull the carb cleaner into the carb. This may dislodge the obstruction. If this doesn't work, you will have to remove and cleant he carburator. Hope this helps.

SOURCE: how do i put the throttle trigger back on stihl

On the carburetor hold the throttle lever to where it would be at full throttle. Then you can put the cable back in the catch for the cable that is on the throttle lever and let go of the throttle lever. The throttle cable should work with the throttle trigger now. The throttle cable only fits one way. The larger hole on the throttle cable catch should go to the ball at the end of the cable.
I hope this helps.
If you feel you cannot do this, then I would take it into your closest Stihl dealer and ask them if they would help you. I do this small repair for free at my shop. Have a good day.