Stihl FS120, won't idle, checked so many things ...

First of, sorry for the long post.

I have this Stihl FS120 for a while (like 7 years) and I have a KM94r too, coming here because i'm out of ideas on how to fix my weed eater.

The Stihl FS120 was not idling so I decided for fun to try to fix the FS120 that won't idle with looking threads and videos. Usually, was able to fix a few things that way (like a microwave, a dishwasher, 2 mowers). Now, the FS120, tt really doesn't want to idle. As soon as I release the throttle, it dies. If you run it full throttle, no problem, it can run all day long BUT, full throttle (or 3/4) only... Of course, tried the low speed screw to enrich and get it leaner, tried the idle screw and also checked the spark arrestor, checked without the muffler, without the air filter, another spark plug, tried to re-align the ignition system, THAN began changing parts!

What has been changed for new...
1st: All fuel line (tank to carb, primer to carb, primer to tank) including fuel filter and the two grommets
2nd: Carburator
3rd: Than... Ordered a kit with air filter, spark plug, the ignition system, also a new gas cap, new gaskets and decided to change everything...
4th: Cylinder and piston and while there, changed the bearings.

I was "cheap" on the cylinder and piston because they looked ok so ordered one from China instead of OEM...

So, what's remaining in this weed wacker that hasn't been changed... The muffler (it's very clean, no spark arrestor in it), the clutch, the flywheel, the shaft itselfs...

So i'm here to get some idea what could be wrong OR what to test!!! It didn't costed a lot to have all those parts and I have a KM94r that makes the job but would still like to get the FS120 to run

A frozen or stuck clutch can cause it to not idle. Does the head spin when you are cranking it....if so clutch need attention. That the only thing I can think of...you look like you covered every other area.

MowLife said:

A frozen or stuck clutch can cause it to not idle. Does the head spin when you are cranking it....if so clutch need attention. That the only thing I can think of...you look like you covered every other area.

Click to expand...

Edit: it doesn't spin when cranking.

Well now you are in deep poo.
The problem with random replacements now everything you have done is suspect

So starting with the cylinder & barrel.
Who did you buy it off & was it branded, either by packaging or cast into the cylinder.

Because we are working with 15 variabes we need to cut things down a bit.
So go buy a red spark tester, search "Spark tester" on here there was a thread a while back about them.
Then set things up so you can clamp the trimmer into some thing solid, vice / pushbike rack whatever it just need to be fixed so the trimmer does not buzz around like mothers little helper.
Set up a camera so it is focused on the spark tester .
IT must be fixed , not held in a hand, but sitting on a shelf with someone steading it will do.
Start the trimmer , warm it up then turn on the video recording and very slowly release the trigger till the trimmer stalls out.
Do this several times and doing it somewhere a bit dark will be a great benefit.
Now go look at the video.
If the tester stops flashing before the engine stops spinning your problem lies in the ignition circuit.
If the tester flashes all the way down to the bitter end then your problem is fuel related.

This is our starting position.

Or we could just keep on guessing till we guess it right

bertsmobile1 said:

Well now you are in deep poo.
The problem with random replacements now everything you have done is suspect

So starting with the cylinder & barrel.
Who did you buy it off & was it branded, either by packaging or cast into the cylinder.

Because we are working with 15 variabes we need to cut things down a bit.
So go buy a red spark tester, search "Spark tester" on here there was a thread a while back about them.
Then set things up so you can clamp the trimmer into some thing solid, vice / pushbike rack whatever it just need to be fixed so the trimmer does not buzz around like mothers little helper.
Set up a camera so it is focused on the spark tester .
IT must be fixed , not held in a hand, but sitting on a shelf with someone steading it will do.
Start the trimmer , warm it up then turn on the video recording and very slowly release the trigger till the trimmer stalls out.
Do this several times and doing it somewhere a bit dark will be a great benefit.
Now go look at the video.
If the tester stops flashing before the engine stops spinning your problem lies in the ignition circuit.
If the tester flashes all the way down to the bitter end then your problem is fuel related.

This is our starting position.

Or we could just keep on guessing till we guess it right

Click to expand...

OK, spark testing is a good idea. If it can helps, as soon as I release the throttle, the engine dies. I'm ok starting from there, i'll go check for a spark tester (always useful anyway), what surprises me is that I have the exact same behavior as with my old stuff (old air filter, old carburator, old gas cap, old cylinder, old fuel lines, old fuel filter, old cylinder, old piston, etc).

The only parts that are not genuine Stihl are the fuel cap, cylinder and piston. The hoses, gaskets, carburator, the ignition/spark plug cable, the Champion plug are from my Stihl dealer. I don't think the fuel line are genuine Stihl though because they were cut out from a big box. The cylinder came as two pieces (cylinder and piston).

Thank's everybody for the tips.

Set the camera up and become a movie star.
I work one up and regularly have to resort to the camera because even when clamped tight into the work vice they still bounce around so much it is hard to see what is happening.

Now if you want to go the whole hog, go one better than the red spark tester and get a color tune ( made by Gunsens , google it ).
I have had mine for over 30 years and is the fall back for things that do not make sense.
Same set up required with hand held engines, still need the video.
However it will show you if you are getting spark & no fuel, too much fuel or too little fuel.

bertsmobile1 said:

Set the camera up and become a movie star.
I work one up and regularly have to resort to the camera because even when clamped tight into the work vice they still bounce around so much it is hard to see what is happening.

Now if you want to go the whole hog, go one better than the red spark tester and get a color tune ( made by Gunsens , google it ).
I have had mine for over 30 years and is the fall back for things that do not make sense.
Same set up required with hand held engines, still need the video.
However it will show you if you are getting spark & no fuel, too much fuel or too little fuel.

Click to expand...

Same day I read your message, I ordered the spark plug tester... I will report back after testing when it stalls

bertsmobile1 said:

Well now you are in deep poo.
The problem with random replacements now everything you have done is suspect

So starting with the cylinder & barrel.
Who did you buy it off & was it branded, either by packaging or cast into the cylinder.

Because we are working with 15 variabes we need to cut things down a bit.
So go buy a red spark tester, search "Spark tester" on here there was a thread a while back about them.
Then set things up so you can clamp the trimmer into some thing solid, vice / pushbike rack whatever it just need to be fixed so the trimmer does not buzz around like mothers little helper.
Set up a camera so it is focused on the spark tester .
IT must be fixed , not held in a hand, but sitting on a shelf with someone steading it will do.
Start the trimmer , warm it up then turn on the video recording and very slowly release the trigger till the trimmer stalls out.
Do this several times and doing it somewhere a bit dark will be a great benefit.
Now go look at the video.
If the tester stops flashing before the engine stops spinning your problem lies in the ignition circuit.
If the tester flashes all the way down to the bitter end then your problem is fuel related.

This is our starting position.

Or we could just keep on guessing till we guess it right

Click to expand...

First of all, thank you again.

Finally got time to check it out yesterday. The spark plug tester light is flashing until the weed eater completly dies so from what you said, it's fuel related :| Well... I have no clue where to start. Tried to put back the old carburator, exact same behavior but I think that's something I tried a few months ago already. Suggestions???

Thank you

Thanks for the spark information.
Does it start with the throttle in the normal position or do you have to hold the trigger wide open for it to start ?
As the engine is dieing does it blow smoke at all , if so what colour ?
When it is dieing try closing the choke a little and see what happens.
Have to set the high speed needle ?
If so how many turns out from lightly seated ?

On the subject of needles, what is the setting on both the H & L needles ( turns out from lightly seated ).
Have you done a compression test, or tested the crankcase seals ?
Bad seals will make the engine hard to start but run ok at high speed but refuse to idle unless the L needle is very rich to compensate for the fuel which leaks past the seals.
A leaking fuel system ( tank , breather or cap ) will again allow the engine to run flat out where venturii effect will suck fuel through but at low speeds the fuel can not get into the carb.
If the carb has a supply & return line then the while system is pressurized to around 7psi while running and without this pressure the fuel pump can not "suck" fuel into the carb.

And again who did you buy the cylinder & piston from ?
Margins in Chinese factories are razor thin so there is no such thing as unsold trash parts.
Whatever the original orderer rejects as being substandard gets sold to surplus wholesalers ( usually with a Hong Kong address ) then on sold through Amazon or Ebay as OEM parts.
You might find this helpful
View attachment Untitled 7.pdf

bertsmobile1 said:

Thanks for the spark information.
Does it start with the throttle in the normal position or do you have to hold the trigger wide open for it to start ?
As the engine is dieing does it blow smoke at all , if so what colour ?
When it is dieing try closing the choke a little and see what happens.
Have to set the high speed needle ?
If so how many turns out from lightly seated ?

On the subject of needles, what is the setting on both the H & L needles ( turns out from lightly seated ).
Have you done a compression test, or tested the crankcase seals ?
Bad seals will make the engine hard to start but run ok at high speed but refuse to idle unless the L needle is very rich to compensate for the fuel which leaks past the seals.
A leaking fuel system ( tank , breather or cap ) will again allow the engine to run flat out where venturii effect will suck fuel through but at low speeds the fuel can not get into the carb.
If the carb has a supply & return line then the while system is pressurized to around 7psi while running and without this pressure the fuel pump can not "suck" fuel into the carb.

And again who did you buy the cylinder & piston from ?
Margins in Chinese factories are razor thin so there is no such thing as unsold trash parts.
Whatever the original orderer rejects as being substandard gets sold to surplus wholesalers ( usually with a Hong Kong address ) then on sold through Amazon or Ebay as OEM parts.
You might find this helpful
View attachment 41948

Click to expand...

It does start properly when when the "trigger" is set on the "start" position.
No smoke at all when dying,
No noticeable smoke when running at full throttle either
The highspeed and lowspeed were tried in many position but basically, when it was running fine it was fully clocked minus 1 turn for the high and low speed
When I changed the piston/cylinder, I had completly removed the bearing and seals and just lubbed them since they were all "as new" but no compression test :|
The damn weed eater I put it in start mode, push the primer 5-6 times, put the choke on, 3rd start, it starts like if it was a new weed eater, even better than my KM94r but it dies as soon as I remove the choke. If I keep it on the choke, it will die if pulling the throttle and with removing the choke, it will start again but if I don't give throttle it will die.

The only thing in your message I think that I didn't tried on the weed eater is turning the choke knob a bit when it dies.

As said in my first post I think, everything comes from the Stihl dealer except the piston/cylinder (farmertec junk, that's my opinion). I tried at first changing all the fuel lines, tried it, not working. Changed the electrical wire, tried it, not working, decided to change the carburator, tried it, exact same problem, then went ahead and changed the cylinder/piston, same problem then I read "if the venting is not good" uh oh, I ordered a new fuel cap since the one there, the gasket was in pieces and removed earlier this year, tried it, not working. Tried the old carburator in a "desperate" try but with same results.

Like my girlfriend said "honey, we usually run it full throttle, it's ok, it's ok" lol (that's what we've been doing for the last 2 years but I got tired of it and bought a KM94r but if I can get the FS120 to work properly, i'll keep it for the circular saw, power on the 120 is definitely better than the 94)