OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters Two Stroke Trimmers & Brushcutters stihl fs120

Have just nursed the old stihl fs120 through the last 12 months, replaced piston /coil/ carby/and other bits and pieces.
Recently it has started playing up again by not reving up after start up I have spent a few hours fiddling with it but no matter how much I adjust the carby no joy .
I have removed the muffler and hey presto the motor revs just fine . before I go and buy a new muffler my question is, do you think that the problem is the muffler or perhaps something else and by removing the muffler it eases pressure off some other faulty part and allows the motor to run just fine? Any advise would be appreciated.
cheers Bill

Hi Bill, and a warm welcome to the OutdoorKing forum...We all like to learn new things and tinker here.
I am sure Global Moderator Grumpy will chime in here and give you some excellent advice....
Hoping you enjoy the forum Bill...
Once again, to the forum,

Hi Bill, I've come across a few trimmers like that and just recently a Kawasaki TH48 post hole digger with the same issue. Some mufflers run a screen(i think they call it a spark arrestor) which generally blocks up and restricts exhaust flow. If the FS120 has a spark arrestor clean it and it should come good. If not, before you rush out and buy a new muffler, try soaking the old one in a suitable solvent,eg carb cleaner, thinners, possibly even fuel for a few hours and blow it out with some compressed air and see if you can clear it up. When all else fails then go and buy the new muffler.

Thanks bigted

I have been soaking the muffler in caustic soda for 8 hours, but when I refitted it to the brush cutter the same problem occurred looks like a new muffler .
thanks for your help much appreciated
cheers Bill

Bill, it is important to identify whether you have a fine mesh screen across the muffler's entry port. That is a spark arrestor. It is required in some places in the US, and it is good practice to use one if you ever operate in forest or grassy areas, especially in summer. The spark arrestor is detachable. If you have any kind of screen or filter across the entry port, just remove it and try running the machine with the muffler fitted, but without the spark arrestor. If that fixes the problem (which it very often does) you then need to spend some time with solvent and a toothbrush, cleaning the screen until you can see through it easily.

If there is no spark arrestor screen, you most likely have a mass of something inside the muffler. There are two common types of mass. One is a wasp's nest, the other is a build-up of oily soot due to the machine having too much oil in its fuel for a sustained period. Solvents don't usually do much to wasp's nests, though they sometimes make them worse. My view is that the best and quickest fix for a mass in the muffler is to heat the whole thing up to a fairly bright red and hold it at that temperature for a while. If it smokes a lot, be encouraged: that shows you needed to do it. Then let it cool off naturally - don't quench it of course - and bang it at various angles on the bench, hoping a lot of loose stuff will fall out of the entry and exit ports.