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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)

Farmer Teck is the retail outlet for the company that makes OEM parts for a lot of factories that assemble big brand chain saws.
The same parts are sold by all of the after market parts suppliers and come with full warranty so are usually good quality.
So if it came in a package labled farmertech it should be in specification.

The stihl service chart says 3 turns out for both needles but that will be for a newish head with EPA compliant carb as they have finer tapers, thus more turns.
The service manual I have specifies 1 turn out on both needles.
Check the impulse holes, they must be clear so the pulses can work the fuel pump.
Stihl used a few different carbs on the 120 heads and I can not remember if they all had impulse hoses or just holes in the manifold.
If holes in the manifold make sure the gasket is not blocking the hole off.
Pull the carb off, turn it sideways and blow low volume low pressure air through the venturii .
You should be getting fuel out the other end .
bes trick is to turn down a compressor then use an air duster inside a dunny roll centre.

And finally about the seals.
They have to be airtight in both pressure & vacuum, looked good does not cut the mustard they have to be tested.
Check Joe Pace Two stroke diagnosis on U tube.

The problem with most of the repair videos is the tech already knows what the problem is so just shows you how to fix it rather than how to find it.

bertsmobile1 said:

Farmer Teck is the retail outlet for the company that makes OEM parts for a lot of factories that assemble big brand chain saws.
The same parts are sold by all of the after market parts suppliers and come with full warranty so are usually good quality.
So if it came in a package labled farmertech it should be in specification.

The stihl service chart says 3 turns out for both needles but that will be for a newish head with EPA compliant carb as they have finer tapers, thus more turns.
The service manual I have specifies 1 turn out on both needles.
Check the impulse holes, they must be clear so the pulses can work the fuel pump.
Stihl used a few different carbs on the 120 heads and I can not remember if they all had impulse hoses or just holes in the manifold.
If holes in the manifold make sure the gasket is not blocking the hole off.
Pull the carb off, turn it sideways and blow low volume low pressure air through the venturii .
You should be getting fuel out the other end .
bes trick is to turn down a compressor then use an air duster inside a dunny roll centre.

And finally about the seals.
They have to be airtight in both pressure & vacuum, looked good does not cut the mustard they have to be tested.
Check Joe Pace Two stroke diagnosis on U tube.

The problem with most of the repair videos is the tech already knows what the problem is so just shows you how to fix it rather than how to find it.

Click to expand...

It was definitely from Farmertec (purchased directly from them) so it's all good then!

I will definitely check for the carburator (remember it was doing the exact same thing with the original carburator). Maybe the the manifold gasket, i'll go check that right now!

I checked the Joe Pace Two stroke diagnosis and it's beyond my knowledge and equipment so I won't be able to check the seals. I guess my FS120 will be used full throttle until I can't stand it anymore and may try to sell it for parts. :|

Lost my other message so this one will be shorter!

Long story short: Before stopping the project, I decided to go through EVERYTHING you said that I was able to do so: Blew air in the carb and fuel came the other end. Then, I saw the hole you were talking about where the manifold is but the hole was blocked by the gasket so went to check the parts diagram and, sad story, I inverted two gaskets (7 and 20). I've put them properly and it's better, the weed eater doesn't die as soon as I release the throttle now but dies after 1 second of idle. I tried a combination of low speed screw (-1 to -1.5 turn) and idle screw. It can idle but at mid speed, something that was not possible before.

If you have any other ideas, go ahead, it helped a lot, i'm just unable to do the leak/seals/compression test. I have another carburator, I may give it a try tomorrow

Don't feel bad about that,
You are neither the first or last person to do that.
So now if you can get it to idle , even if it is very fast, set the idle screw so the engine says running then adjust the L needle to get the best idle
Readjust the H needle then try moving the Idle stop out a bit further, till the engine sounds like it will die.
Adjust the L needle , then pull the trigger in fully & do the H needle again.
Repeat till you get a smooth idle.
Because the carb always pulls fuel from both the H & L jets, every time you adjust one it affects the other.
Fuel always flows through the L jet from Idle right up to WFO.
But because the H outlet is a lot bigger, it needs a lot more air rushing past it before it can suck fuel through.

There is a lot of different opinions abou this bu I have always sdjusted a needle lean till the engine starts to miss then rich till it starts to miss again then turned the needle to a point just on the rich side of 1/2 way between the two sports when the engine faultered.

Examine the taper ends of the needles under the strongest magnifier you can find.
If there is the slightest amount of hooking or the smallest ridge then toss the needle & get a new one.

bertsmobile1 said:

Don't feel bad about that,
You are neither the first or last person to do that.
So now if you can get it to idle , even if it is very fast, set the idle screw so the engine says running then adjust the L needle to get the best idle
Readjust the H needle then try moving the Idle stop out a bit further, till the engine sounds like it will die.
Adjust the L needle , then pull the trigger in fully & do the H needle again.
Repeat till you get a smooth idle.
Because the carb always pulls fuel from both the H & L jets, every time you adjust one it affects the other.
Fuel always flows through the L jet from Idle right up to WFO.
But because the H outlet is a lot bigger, it needs a lot more air rushing past it before it can suck fuel through.

There is a lot of different opinions abou this bu I have always sdjusted a needle lean till the engine starts to miss then rich till it starts to miss again then turned the needle to a point just on the rich side of 1/2 way between the two sports when the engine faultered.

Examine the taper ends of the needles under the strongest magnifier you can find.
If there is the slightest amount of hooking or the smallest ridge then toss the needle & get a new one.

Click to expand...

Taking the time to say thank you.

For sure when I've put back everything together, I inverted the two gaskets shown in my previous picture and it was not working because of this so I came here asking for help not knowing what could be wrong.

When I disassembled everything though, the order was ok and things were not working properly so for sure there was something bad other than just the carburator because the first time, I changed only the carb without touching anything else and I had the problem. Unfortunatly, in my next step I changed the coil, all fuel lines, the air filter, fuel filter and primer bulb, I don't remember if I removed the carb again and the gaskets but was still not working and then, I changed the piston, rings, cylinder, than after that, seeing it was still not working, changed the fuel cap and then as said, came here asking for help!

It's unfortunately for all those changes that I will never know what was the faulty part but for sure, not all this had to be changed. Was it the piston, the cylinder, the bearings, the coil, the tubing, the fuel cap, the primer bulb, the spark plug (nah, I think it's the first thing I changed lol), the air filter or the fuel filter, I will never know because the gasket was preventing air to go inside the cylinder at one point of my replacements so again, thank you for everything!

It now idles fine, works great, definitely a lot more torque and power compared to my new KM94r.

Check out "Steve's small engine saloon" on Youtube.Guaranteed he has a vid for your problem.