FS120 trimmer repair

[kyle1!]

I bought a used trimmer this weekend. I had no trouble starting it the day I bought it and when I got home that day but the next day I had trouble getting it to start. After pulling a bunch of times on choke/hitting fuel prime bulb 4 or 5 times the spark plug is still dry. I got it to start by dribbling some gas in the cylinder. Once it was running I was able to use it for 10mins and it ran as expected. Sat it down to change the string and couldn't get it start and the fuel prime bulb split on a push so I was done.

Where do I start with this thing? I assume replace the fuel filter and prime bulb first? Or just go through the whole fuel system while I have it apart? Looks pretty simple to take apart.

 

[DND 9000]

It would be good to make a complete service. That means fuel system cleaning and rebuild. Also check for spark when it is hot and does not start. Maybe the coil is not working properly when hot. Also a pressure and vacuum test would be good to do, that everything is tight.

 

[John Lyngdal]

My FS120 for some reason is more difficult to keep in tune than my FS250. Don't know why, that's just the way it is.
I'd pull the carburetor and check the fuel inlet screen for junk and the metering diaphragm for alcohol abuse.
I'd also fill the tank to the brim with fuel and see if you can get the priming bulb to fill.
If it doesn't fill, check and see if the fuel line connections to the bulb are reversed.
If the lines aren't reversed and you can't get the bulb to fill, the bulb is defective or you have a leaky fuel line.

 

[kyle1!]

I did not have problems getting the bulb to fill. It developed a pin hole leak but that happened after I was trying to get it to start. I took the cover off last night and the lines to the primer bulb were hard/brittle and the small one going to the top of the carb did not seal tight. Looks like someone has been in the carb because it is spotless compare to the rest of the machine. The fuel line is green and appears new. I thought stihl oem fuel line were black? I will replace the fuel system stuff and see where that gets me. Thanks for the help.

 

[stihl86]

I did not have problems getting the bulb to fill. It developed a pin hole leak but that happened after I was trying to get it to start. I took the cover off last night and the lines to the primer bulb were hard/brittle and the small one going to the top of the carb did not seal tight. Looks like someone has been in the carb because it is spotless compare to the rest of the machine. The fuel line is green and appears new. I thought stihl oem fuel line were black? I will replace the fuel system stuff and see where that gets me. Thanks for the help.

Leave the green line. It is Stihl and will outlast you and me both. Its a nitrile material that seems to be impervious to anything and everything.
Only weakness is cold. They used it in some saws but it didn't work out due to that reason

 

[kyle1!]

Thanks for that.

 

[Ax-man]

Check the ignition module for spark after it gets warmed up . I have been fighting with my 120 for the last 2 to 3 years with the same symptoms . I blamed it on the carb but after buying a new carb recently it worked flawlessly till I went to use it recently . Started right up but no acceleration on the high end ,retuned and it worked till I shut it off and wouldn't restart . This time I checked the module and got a hit and miss spark.

I know this is an old thread and the OP probably has solved his problem but the 120 is a nice trimmer when they run right and I hate to junk the one I have even though I only use it around the house and patio with one of those poly cut heads . Stihl also has a little 3 blade brush knife for that trimmer and I like using that in the rough areas and going around trees .

 

[kyle1!]

I am just lazy about fixing this trimmer. Drought conditions have meant no grass growing so I have not been in any hurry to fix it. Replaced the primer line from tank to bulb and tried to install the new primer bulb into the housing but I can't get it to go. It seems like the prongs on the bulb are too far apart. Placed a socket over the bulb to hopefully force it but no go. It should just snap in without much effort. Do I have the wrong bulb, Part # 0000-350-6201 for the FS120?

 

[DND 9000]

You maybe have the wrong primer bulb. There was a change in 2009, where the geometry of the pump bulb and the air filter housing where it snaps in was changed.

left: older version air filter housing
right: newer version air filter housing

left: older version with straight connectors
right: newer version with hose barb connectors

 

[kyle1!]

The primer bulb and the housing from the machine are the older versions. My new primer bulb is the newer version with the barbs off set. Looks like I need #4130 350 6200 Thanks DND

 

[michael j]

Same problem with one. Put a coil on it.

 

[kyle1!]

I am really lazy. Finally put a new coil and a new ngk R plug old one was a champion in the unit. The unit runs strong and starts like it should. Thanks everyone.