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Fuel starvation

#1

A

abarone300

My FS 120 R after having been stored for the winter, this time (in the past when this happened, I would struggle with it, but finally it would start, not this time) it would not start! Found out; that the muffler was clogged (some bees had gotten inside via the opening) After installing a new muffler the grasscutter would idle fine after brought to temperature, but when I pulled on the accelerator it would choke and shut down; when switching the knob to "choke" it would accelerate wiithout choking and without shutting down; but of course would not run well and not reach maximum speed since it is set on the "choke" position. Can somebody tell me if this is a classic case of fuel starvation and how I can resolve the problem?

ps - no changes have been made to the carburator's settings, not even the idle speed screw! I've tested the cutter without a muffler installed, and no "choking" has been experienced, since it runs and cuts fine, but of course I can't run it without a muffler installed...


#2

StarTech

StarTech

Most likely the metering diaphragm in the carburetor has stiffen and needs replacing.


#3

Fish

Fish

Put up the numbers off of the carb. Just richen the mixture screws a tad.


#4

A

abarone300

Fish said:
Put up the numbers off of the carb. Just richen the mixture screws gas a tad.
Click to expand...
Good morning Fish, thank you for your answer. I've followed Startech's advise and ordered a seal kit (includes a new diaphragm). You think enriching the fuel mixture will resolve the issue, I really need to use the cutter to cut my grass now, can't wait for the kit to arrive and don't feel like taking the carburator apart to replace the seals if I don't have to ( at least for now, i probably will have to replace them later...); because the grass it's over running my place!
Do you think the diaphram has deteriorated to a point that it prevents enough gas to go though? Thanks


#5

Tiger Small Engine

Tiger Small Engine

abarone300 said:
Good morning Fish, thank you for your answer. I've followed Startech's advise and ordered a seal kit (includes a new diaphragm). You think enriching the fuel mixture will resolve the issue, I really need to use the cutter to cut my grass now, can't wait for the kit to arrive and don't feel like taking the carburator apart to replace the seals if I don't have to ( at least for now, i probably will have to replace them later...); because the grass it's over running my place!
Do you think the diaphram has deteriorated to a point that it prevents enough gas to go though? Thanks
Click to expand...
The only way to know for sure, is to adjust carburetor and see if that helps. If not, step two is to remove carburetor, clean, replace metering diaphragm, and install carburetor. Make sure fuel is no more than 30-60 days old.


#6

A

abarone300

OK, I started making the adjustment. Before I realized that the screws can only be turned 3/4 of a full turn, and I ended up over adjusting them! My "tad" wasn't quite your "tad".
Now I can't even start the thing now; because Fuel and Air are way out of adjustment (I suppose they need to be in sink too, and I suppose the engine temperature has to be considered). What is a good starting point, a rough screw setting before fine tuning the two screws, so I can at least start it?
The markings I think are in German: "LA" for idle screw (it's the only screw I'm sure of) and "H" and "L" ; I suppose these two screws are FUEL and AIR? In the mean time I'm going to consult the manual, maybe it has the English translation. Thanks

PS - before I over adjusted the screws, I got it to run pretty good (it was revving ok, except for a small bugging (that's when I exaggerated and messed up)


#7

kbowley

kbowley

Take the carb off and get the numbers off it to order the correct kit or take the carb to a dealer and have them install the new carb kit. You may have damaged the adjustment screws. Initial adjustment is 1 1/2 turns out after lightly snugging them up. Regarding the letters beside the adjustment screws, H is high-speed, and L is the low-speed adjustment.


#8

A

abarone300

Thanks kbowley, I had it all wrong then:"H" for high speed and "L" for low speed! Is this how all mixture screw work on all carburators; even car's? As far as I know the regulating screws are not damaged, I was veri careful not to overtight them and. And I can only turn the screws 3/4 of a full turn (about 270°)


#9

Fish

Fish

The "L" is the low speed mixture screw, and turning them to the left richens it up. The "H' is the one farthest from the engine, and it richens the same way. There are usually limiters that you cannot richen them and further without removing the limiter caps. So turning them both left to the stop will likely get you back to where you were.


#10

Fish

Fish

I tried uploading the manual but it is too large.
P.M. me if you want me to e-mail it to you.


#11

A

abarone300

Fish said:
I tried uploading the manual but it is too large.
P.M. me if you want me to e-mail it to you.
Click to expand...
It's ok fish, I think I have the manual in a box somewhere, the machine is only a couple of years old Thanks.


#12

Fish

Fish

abarone300 said:
It's ok fish, I think I have the manual in a box somewhere, the machine is only a couple of years old Thanks.
Click to expand...
The technician's repair manual? This manual is good to have.


#13

A

abarone300

Fish said:
The technician's repair manual? This manual is good to have.
Click to expand...
No, I was talking about the user's manual! But I'll lookout for the PDF version of the service manual. Thanks Fish!


#14

A

abarone300

At this point in time, I'm still having an issue with "L" and "H" set screws (more with high revs). After a good rest I was able to restart the machine, removed the limiter cap (I'm going to probably end up buying a new carburator anyway...), turned the screw clockwise, to see how many turns it would take to reach the limit (It took a little over 1 and 1/2 turns) then screw back to the original setting and proceeded in turning the screw gradually in both directions, with very little change in RPM. I definetely couldn't get it to rev at a workable high revving cutting speed ( at this speed I suppose I can only "comb" the grass)! Also, will mention that when I had the accelerator pressed all the way, the motor almost stalled, when I adjusted the screw almost to the limit. Would you guys know what a Stihl or a good substitute carburator goes for? I'm weighing in the options whether to buy a new carburator or install a seal or rebuilding kit . I still haven't received the seal kit I ordered, as far as I know it might be coming all the way from China; Amazon.it doesn't always ( in fact they never do, and you assume you'll have it in 2-3 days) tell you where it's coming from, before you purchase the stuff.


#15

Fish

Fish

I'll upload the manual on the Stihl Chainsaw fans' Facebook page, just go to the file section.


#16

A

abarone300

Fish said:
I'll upload the manual on the Stihl Chainsaw fans' Facebook page, just go to the file section.
Click to expand...
Thanks for your help!


#17

StarTech

StarTech

Very unusual to find one online.

Download Stihl FS 120 Service Manual | ManualsLib

www.manualslib.com www.manualslib.com


#18

kbowley

kbowley

abarone300 said:
At this point in time, I'm still having an issue with "L" and "H" set screws (more with high revs). After a good rest I was able to restart the machine, removed the limiter cap (I'm going to probably end up buying a new carburator anyway...), turned the screw clockwise, to see how many turns it would take to reach the limit (It took a little over 1 and 1/2 turns) then screw back to the original setting and proceeded in turning the screw gradually in both directions, with very little change in RPM. I definetely couldn't get it to rev at a workable high revving cutting speed ( at this speed I suppose I can only "comb" the grass)! Also, will mention that when I had the accelerator pressed all the way, the motor almost stalled, when I adjusted the screw almost to the limit. Would you guys know what a Stihl or a good substitute carburator goes for? I'm weighing in the options whether to buy a new carburator or install a seal or rebuilding kit . I still haven't received the seal kit I ordered, as far as I know it might be coming all the way from China; Amazon.it doesn't always ( in fact they never do, and you assume you'll have it in 2-3 days) tell you where it's coming from, before you purchase the stuff.
Click to expand...
Hi, you didn't mention which screw you adjusted, IE, H or L. Which one did you remove the limiter from? If it doesn't rev up, you need to open the H screw further until it runs clean. then back to the L screw to set so it doesn't bog when you goose the throttle. You need to work with both to get a good idle, good wide open, and no bogging when goosing the throttle from idle. Turn them slowly as some carbs are sensitive when fine tuning to get the result. Please ask if you need any assistance. However, it likely needs a carb kit. A local Stihl dealer should have the correct carb kit for it. Is it a Zama C1q s161b? The carb kit should be an RB-83.


#19

A

abarone300

kbowley said:
Hi, you didn't mention which screw you adjusted, IE, H or L. Which one did you remove the limiter from? If it doesn't rev up, you need to open the H screw further until it runs clean. then back to the L screw to set so it doesn't bog when you goose the throttle. You need to work with both to get a good idle, good wide open, and no bogging when goosing the throttle from idle. Turn them slowly as some carbs are sensitive when fine tuning to get the result. Please ask if you need any assistance. However, it likely needs a carb kit. A local Stihl dealer should have the correct carb kit for it. Is it a Zama C1q s161b? The carb kit should be an RB-83.
Click to expand...
Hi Kevin, thanks for the help.Before I contacted this forum, the only screw I adjusted was the "IE" to set the idle speed a little higher. After that; before I order the seal kit I tried to adjust "H" as a last resort. As I mentioned before, the adjustment didn't work (the engine would stall before it reached the extreme right). I went on and ordered the Seal Kit (which included four seals, one of them being the diaphram).
Received the kit and installed it, but found out the seal kit did not included a fifth seal I ended up needing (it got destroyed when I tried to remove it).
In short, the carburator still off, because I haven't been able to find the two outside seals (one of them is the one I ripped). It's funny how all the stores online in Italy have the seals in "stock", but it's going to take 2-3 weeks to get from their warehouses; it's definetly coming from China ,for sure! (I'm trying to avoid ordering stuff from China, unless I really have to).
I'm still looking for the seals, but I'm sure I'm going to have to either order it from China or make one of the two ( the second one it's not a critical seal and can use the old one)


#20

A

abarone300

abarone300 said:
Hi Kevin, thanks for the help.Before I contacted this forum, the only screw I adjusted was the "IE" to set the idle speed a little higher. After that; before I order the seal kit I tried to adjust "H" as a last resort. As I mentioned before, the adjustment didn't work (the engine would stall before it reached the extreme right). I went on and ordered the Seal Kit (which included four seals, one of them being the diaphram).
Received the kit and installed it, but found out the seal kit did not included a fifth seal I ended up needing (it got destroyed when I tried to remove it).
In short, the carburator still off, because I haven't been able to find the two outside seals (one of them is the one I ripped). It's funny how all the stores online in Italy have the seals in "stock", but it's going to take 2-3 weeks to get from their warehouses; it's definetly coming from China ,for sure! (I'm trying to avoid ordering stuff from China, unless I really have to).
I'm still looking for the seals, but I'm sure I'm going to have to either order it from China or make one of the two ( the second one it's not a critical seal and can use the old one)
Click to expand...
I have finally reinstalled the carburator after using the new internal seals and and the two external seals I made. Tried to start it and its a NO GO still!
I'm going to try one more thing and then give up and order a new carburator!
Want to make sure the "L" screw is adjusted at a "ball park" position to exclude that it's preventing the start up process (I assume the "H" screw doesn't effect this process, and I won't worry about it for now)
Will let the cutter sit for a while , check the position of screw "L" a then try it again. If it doesn't start I'll go ahead and order a new carburator. Thanks to all:


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