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My Stihl FS 120/R after having been stored for the winter, won't start. It will start if I remove the spark plug and inject a bit of gasoline
in the cylinder, but after the fuel runs out it shuts off!
I thought I had the issue resolved, by disassembling and blowing the carburettor with compressed air, it tested fine! But after putting it
back together (the covers and cutter rod/clutch unit), it went back doing the same exact thing!
Can you give me some pointers on what to check?
I've tested the priming bulb and it seems ok, it's is always full of fuel, (though, I'm a little puzzled; I always thought it pushed fuel
into the carb when pumped, instead it pulls OUT fuel, is it basicly a return line to the tank, is it the way it's supposed to work?)
Which of the two lines (tubings) attached to the fuel tank is the return line, is it the one leading to the primer bulb and going to the RIGHT of the carb?
Which of the two lines (tubings) attached to fuel tank is the supply line, the one leading to the carb and entering on the LEFT Side of the carb?
How do I test the carb if it's pulling fuel out of the fuel tank?
How does the carb replenish itself with fuel, it doesn't seem to have a fuel bowl or a float, does it pull fuel from the tank and
inject it directly into the thottle body? I appreciate your patience and any help you might give me. Thanks

The primer bulb should pull from the tank and produce a small squirt of fuel into the carb throttle body you should be able to see and feel this action , if not it's probably sucking air .Once the machine starts there is a diaphram type of fuel pump which sees a pulse from the crankcase action and keeps fuel flowing to the carb. Check all your fuel lines they can dry rot and crack over time.

If you used gas stored in the mower, I'd start with draining the gas and replacing it with new.

Reply to Bertrrr; I realize that what he's saying makes sense, and that's how I always thought the priming pump worked. This is how I discovered my cutter's carb and pump ìt's setup (to my surprise) and I'm not sure this is the way it's supposed to work:
1) The pump's line does NOT draw fuel from the tank it is infact connected to the RETURN line (the two tubings and the respective nipples will fit NO other way, because different in size).
2) The pump WILL infact draw from the carb, if the tubings are inverted in the pump to push fuel; the bulb CANNOT be pressed (the tubing on the RIGHT SIDE of the carb CANNOT receive fuel) fuel CAN ONLY be drawn by the pump and sent back to the TANK's RETURN line. The two points do not coincide with what Bertrrr said about the pump/bulb and how they should work.

I don't know at this point if this setup, it's how is supposed to work, but it's definetly how the tubings where routed and to my knowledge how the RS 120R was setup when it worked.
In regard to the internal diaphram Bertrrr is talking about and it's ability to suck fuel by way of the crankcase, I haven't been able to test and accertain how this is done and if it's working.

In reply to slomo:
The fuel tank has been drained and fuel been changed. And note that when fuel is added to the cylinder motor will run (for about five seconds or until the 2cl of fuel runs out)

Use fuel stabilizer year round. Why? Because I forgot what I had for dinner last night. How old was this fuel??? Never have to worry about it again. Starts on the first pull isn't bad either.

Just disassembling a cube and cleaning without installing a carburetor repair kit will usually not work. Diaphragms tends to stiffen over time and must be replaced. Once these rubber coating cloth diaphragms are exposed to fuel they start deteriorating over time. Doesn't matter if you are using Ethanol blend or straight Gasoline based fuel/oil mix; both will cause the same problem over time.

And please don't do what some idiot did here. They tried resealing with RTV and it clogged up a prefect good carb.

StarTech said:

Just disassembling a cube and cleaning without installing a carburetor repair kit will usually not work. Diaphragms tends to stiffen over time and must be replaced. Once these rubber coating cloth diaphragms are exposed to fuel they start deteriorating over time. Doesn't matter if you are using Ethanol blend or straight Gasoline based fuel/oil mix; both will cause the same problem over time.

And please don't do what some idiot did here. They tried resealing with RTV and it clogged up a prefect good carb.

Click to expand...

Thanks for your advice StarTech, I'm in the process of ordering a carb kit. If it shouldn't resolve the problem I will order a new Carburetor.

I wouldn't bother with a carb kit, replacement carbs are cheap enough for an aftermarket copy & it saves a lot of time, especially if you go down the full rebuild (welch plugs etc) route.
The original carb will undoubtedly be a superior quality, but for the time saved you can almost count the carb as disposable.
If you repair your existing one, you may find it needs trimming/fettling/adjusting (depending on your preferred term) which can be a nightmare in itself. Also check the exhaust is clear - the flame trap can become blocked & that will cause running issues.

Thank you Jones136, I haven't ordered the kit yet, but I'm getting desperate though, the grass is growing to an alarming rate; it has advanced and swallowing my house like a "Blob"! (I live in the middle of nowhere, but there's lot's of grass around me), have to make a decision before the week is over and decide whether to order a carburator or the kit. I got it to run, but it's still hard to start and it shuts off often, I ran out of ideas.
But you mentioned the muffler and gave me an idea; I'll remove the muffler and see what happens. if it doesn't work after that, I'll go ahead and get a new carburator, I'm tired messing with it!

abarone300 said:

Thank you Jones136, I haven't ordered the kit yet, but I'm getting desperate though, the grass is growing to an alarming rate; it has advanced and swallowing my house like a "Blob"! (I live in the middle of nowhere, but there's lot's of grass around me), have to make a decision before the week is over and decide whether to order a carburator or the kit. I got it to run, but it's still hard to start and it shuts off often, I ran out of ideas.
But you mentioned the muffler and gave me an idea; I'll remove the muffler and see what happens. if it doesn't work after that, I'll go ahead and get a new carburator, I'm tired messing with it!

Click to expand...

Did you check and confirm fuel going thru the tiny screen when carburetor is in your hands? If the screen is partially blocked or blocked, fuel delivery will be the issue. I see lots of handheld equipment with fuel delivery issues that are a result of this blocked screen.

You are the MAN Jones136! You hit the proverbial nail right on the head! You nailed it, the Muffler WAS clogged!!!
Problem solved, I just have to buy a new muffler, I wanted to use the old one in the meantime so I could use the cutter until I got the new muffler in, but it's clogged so bad it reverts to stalling again and I can't leave the cutter without a muffler not only because it's loud (no one around) but for some reason it over revs when I give it gas ( I didn't mess with the carb's adjustments and the gas cable is not getting stuck!) It might have been adjusted from factory with the muffler in place, what do you think? Will see how it acts when I get and install the muffler.
In all the years I've worked with motors (as an amature of course) I have never actually seen an engine failing to start because of a clogged muffler, it never crossed my mind to do it, also because the cutter is less than two years old! But I have a pretty good idea what could have caused the clogging:
As I mentioned earlier I live in the middle of nowhere, but there's not only lots of grass, there is also lots of insects including BEES, lots mice, lots o snakes, you name them, I have them! And where I stored the "grass cutter" it's not exactly a clean enviroment and free of little spyders and "things", and I think the "culprit" the BEES! Anyhow, thanks again Jones136 and to all you guys, you where a great help indeed!

Aldo
PA Italy

Tiger Small Engine said:

Did you check and confirm fuel going thru the tiny screen when carburetor is in your hands? If the screen is partially blocked or blocked, fuel delivery will be the issue. I see lots of handheld equipment with fuel delivery issues that are a result of this blocked screen.

Click to expand...

Thank you Tiger Small Engines!

Hello guys, wanted to update you on the muffler. I was able to unclog it with compressed air and some heavy banging to the ground (those pesty BEES...) enough to start and run the motor on idle, but not enough to run smoothly (on accelleration) and at times it WILL cause the motor to stall, so I decided to go ahead and order a new muffler although not inexpensive, an original it's 35 euro (better than 80 to 100 euro for new carburator, I guess...) I have to order a new spring for the clutch anyway, a word of advise: don't run your engine without the cutting/stalk assembly on; it will elongate the spring that's supposed to close the centrifugal arms and leave the clutch in a position to pick up and spin the cord even on idle, I've done that twice already, never learn! Am I the only person who does that?
Do you guys know if the spring is available without having to buy a whole new clutch assembly? Thanks

Look at your carburetor, if it has a tiny metal cup on the bottom with a wire attached, take the plastic handle of a screwdriver and lightly tap the cup while turning the key. There's a small pin inside the cup that blocks gas to the carburetor. Turning the key magnetically opens that pin and lets gas into the carburetor. You could simply turn off your gas, remove the wire a take the cup off and clean it. If you go that route, do it over a small ceramic dish, there are some small parts. The pin and the spring are small, watch the order they come out. There is probably gunk inside the cup that won't let the pin move. I just fixed the same problem n mine. If your carburetor is original put it aside and save it if you do replace it. There may be a kit to repair it, any replacement will most likely be a cheaper knock-off. The OEM part from the manufacturer is usually several hundred dollars vs something from Amazon that costs $30 to $60 bucks.

Thanks Boomer49, that was the very first thing I checked back when I discovered I had a starting problem:
Tested the fuel lines, and all checked fine. Like I said in the earlier post, the problem has been resolved; in fact I used the cutter for about a half hour, wearing ear muffs of course, didn't have to worry about neighbors; I don't have any! (you guys must be envious of that...)
It turns out though, that after a while, it must have overheated, because the motor woud shut off, don't know if running it without a muffler had anything to do with it ; but I'm going to play it safe and wait for the new muffler and investigate after the muffler installation, if the problem if it still exists.


PS
I'd be curious to know if any of you guys have ever used the mower without a muffler installed; are there after effects other than being very loud and bothering the hell out of the neighbors? Did anybody experience any overheatng or shut off problems and most important can it damage the engine running it without a muffler?
I have the suspicion also; that the carburator's final adjustments are made with the muffler installed at the factory and somehow if the the mower is ran without it, something is bound to happen

I sort of cheated and got this online, it's stuff I know but it explains it with at least a thousand fewer words. It's from HouseToolkit.com , "
Engine Performance and Damage:

• The muffler not only reduces noise but also plays a crucial role in controlling exhaust flow and back pressure.
• Without a muffler, the engine may experience increased back pressure, leading to reduced power and potentially causing damage to engine components.
• The absence of back pressure can lead to an imbalance in the exhaust system, causing undue stress on engine valves and the exhaust manifold, which may result in premature wear or failure2

abarone300 said:

Thank you Jones136, I haven't ordered the kit yet, but I'm getting desperate though, the grass is growing to an alarming rate; it has advanced and swallowing my house like a "Blob"! (I live in the middle of nowhere, but there's lot's of grass around me), have to make a decision before the week is over and decide whether to order a carburator or the kit. I got it to run, but it's still hard to start and it shuts off often, I ran out of ideas.
But you mentioned the muffler and gave me an idea; I'll remove the muffler and see what happens. if it doesn't work after that, I'll go ahead and get a new carburetor, I'm tired messing with it!

Click to expand...

There is an impulse tube from the crankcase to the carb that uses pulses from the crankcase to pump fuel to the carb.These impulses operate the diaphragm in the carb,so make sure the impulse tube is not cracked,clean, and not leaking air.This can cause the exact problem you describe.

Thanks Boomer. Hope there was no harm done, I will not use it untill I get the muffler in (should be here by the middle of nextweek), the one I order previous to this one (from Amazon) it turn out it's coming from China , at the end the month (May!) Amazon forgot to mention that bit of information, so I had to reorder it from a store locally, (for an addition al 11 extra euro for an original) I ordered the first one to save time, instead I ended up getting screwed! Usually Amazon gets you the part pretty quick and get's it delivered in 1 to 3 days, go figure...

closecut said:

There is an impulse tube from the crankcase to the carb that uses pulses from the crankcase to pump fuel to the carb.These impulses operate the diaphragm in the carb,so make sure the impulse tube is not cracked,clean, and not leaking air.This can cause the exact problem you describe.

Click to expand...

Well, in my case I'm pretty sure the bees caused my problem, I will know for sure once I install the new muffler...

I would suggest you start using non-ethanol (E0) gasoline.
Since I began using it several years back, I've never had problems with any of my lawn equipment starting or my carburetors having trouble after sitting all winter.
I've also never needed fuel stabilizer since making the change either.
I believe the E0 gasoline has a longer shelf life.
Here's some places that sell it at the pump.