FS 240 R not idling after start

[Bwcland]

Hello Stihl Forum, FS 240 R we use has thousand of hours, we maintain it regularly. Recently the trimmer will start fine as normal, but the engine stays at 80-90% throttle and will not calm down to idle. Checked the carb adjustment, the throttle cables, choke, and throttle trigger, all appear to be working and moving as normal. Any tips please on how to trouble shoot this? Thanks!

 

[Tiger Small Engine]

Hello Stihl Forum, FS 240 R we use has thousand of hours, we maintain it regularly. Recently the trimmer will start fine as normal, but the engine stays at 80-90% throttle and will not calm down to idle. Checked the carb adjustment, the throttle cables, choke, and throttle trigger, all appear to be working and moving as normal. Any tips please on how to trouble shoot this? Thanks!

Obviously something inside the trigger mechanism or throttle cable or throttle butterfly on carburetor is hanging open. 80-90% of wide open throttle is really fast, so recheck everything. Great trimmer by the way.

 

[Bwcland]

Thank you for response @Tiger Small Engine, checked trigger mechanism, throttle cable, these are good. Need to read about throttle butterfly on carburetor. Any tips for checking this? It is the original carb, 8 years old, thousands of hours and I have not serviced it previously. Any other tips I can check with the carburetor? Disassembling trimmer carburetors somtimes I seem to do more harm than good. With my limited carb rebuild skill set, may be better to put on a new one with new gaskets.

 

[Bwcland]

Thank you for response @Tiger Small Engine, checked trigger mechanism, throttle cable, these are good. Need to read about throttle butterfly on carburetor. Any tips for checking this? It is the original carb, 8 years old, thousands of hours and I have not serviced it previously. Any other tips I can check with the carburetor? Disassembling trimmer carburetors somtimes I seem to do more harm than good. With my limited carb rebuild skill set, may be better to put on a new one with new gaskets.

@Tiger Small Engine ok I see some tips on youtube how to check the throttle butterfly, will give this a go, thanks for the tips

 

[Bwcland]

Hello @Tiger Small Engine I was wrong about youtube, could not find anything on this topic. You are spot on the throttle butterfly is staying open. Not sure why, I noticed the throttle lever P# 4147 121 3500 is not rotating back when release the trigger. Is this motion of throttle lever controlled by the spring on the back side of carburetor? Choke is working ok.
Attached a few pictures with red arrows. Any tips would be greatly appreciated. Thank you.

 

[Tiger Small Engine]

Hello @Tiger Small Engine I was wrong about youtube, could not find anything on this topic. You are spot on the throttle butterfly is staying open. Not sure why, I noticed the throttle lever P# 4147 121 3500 is not rotating back when release the trigger. Is this motion of throttle lever controlled by the spring on the back side of carburetor? Choke is working ok.
Attached a few pictures with red arrows. Any tips would be greatly appreciated. Thank you.

It is hard to say without actually working on the trimmer. Try lubricating the throttle cable. You may need to open the throttle plastic cover up and see if anything is hanging up inside.

 

[Bwcland]

Understood. Opened throttle cover everything ok in there. When machine starts and goes full throttle, when push the throttle lever with flat head screw driver the throttle goes down to idle. Same with the opposite side of the carb, the lever with the spring and where the cable pin attaches, when nudging the lever with flat head it goes to idle easily. Does not need much of a push, its moves easily. Will disassemble carb and see if there is some debris or something in the spring area keeping from allowing the throttle lever to move to idle.