Chainsaw Repair

8 out of 10 of these carbs can be revived if you open both ends, remove H & L screws, the fixed jet, the metering items and the strainer.
Then soak them overnight in an emulsion of kerosene, coconut oil and laundry detergent.
That will looosen up the muck in the passages and the jet valve.
Use water at about 40psi to wash out the carb completely.
Use a low pressure blower to dry it out.
Re-assemble it.
Instead of 2 stroke oil use a 1:1:1 mix of two stroke oil , coconut oil and nitro cellulose thinner.
Use this in a 25:1 fuel mix
Turn out H and L 2  or 2 1/2 turns. Turn in idle screw to its stop.
Start it up.
Blip throttle once in a way.
As engine runs smoothly, turn out idle screw and turn in L & H screws. Until you reach the normal settings.
This has worked  most of the time, all through 2015 and 2016 for me.
Zamas on Stihl brushcutters, a real pain!
120/200/250 are manageable up to a point.
If you want a real pain you should see the carb options for the 4137 series (FS85/80 etc)

My brother imported some parts from an aftermarket supplier, including carbs (non Zama), they were cheap and worked fine. For their price, it would be worthwhile buying them just for the diaphragms and other parts
http://vastfine.en.alibaba.com/product/902461535-218594368/Quality_brush_cutter_parts_carburetor_fits_FS120_200_250.html