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

[bertsmobile1]

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.

 

[CoOlSlY]

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.

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. :|

 

[CoOlSlY]

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

 

[bertsmobile1]

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.

 

[CoOlSlY]

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.

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.

 

[mikehouse]

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