After having kept the machine in storage for a few months, it won't start

[abarone300]

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

 

[Bertrrr]

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.

 

[TonyPrin]

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

 

[abarone300]

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)

 

[slomo]

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.

 

[StarTech]

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.

 

[abarone300]

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.

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.

 

[abarone300]

Would someone have a link with image/s that would show the routing of the fuel tubings from the fuel tank to the pump and carburator (Stihl RS 120 R). Thanks, would really appreciate it

 

[Jones136]

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.

 

[abarone300]

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!