Adjusting Stihl carb - advice needed please!

[Treewolf]

A little while ago I inherited a Stihl FS55 petrol strimmer and over the weekend I thought I'd see what if anything it could do.

 

However I was having trouble getting the carb adjusted right, and my main problem was that the carb shown in the manual (downloaded from a Stihl website) is nothing like the carb on the cutter. The manual shows the conventional three-screw carb, but the cutter has only a single screw (with an LH thread) in the area where usually there are three, and a further screw (Torx socket head) just on the cylinder side of the plastic guide for the throttle cable.

 

Randomly fiddling with these two screws got the machine running and cutting quite well, but the idle speed is way too fast. I can't seem to slow down the idle without also killing the cutting speed and power.

 

Can anyone offer any advice on tuning a carb of this type?

 

Sadly I have little knowledge of the history or age of the machine, or whether in the past it's had replacement parts fitted. Genearlly it seems to be little used and in pretty good condition, certainly too good just to chuck away. My intention if I can get it running properly is to keep it as a spare machine.

 

Thanks.

[Eddie@aspen]

A little while ago I inherited a Stihl FS55 petrol strimmer and over the weekend I thought I'd see what if anything it could do.

 

However I was having trouble getting the carb adjusted right, and my main problem was that the carb shown in the manual (downloaded from a Stihl website) is nothing like the carb on the cutter. The manual shows the conventional three-screw carb, but the cutter has only a single screw (with an LH thread) in the area where usually there are three, and a further screw (Torx socket head) just on the cylinder side of the plastic guide for the throttle cable.

 

Randomly fiddling with these two screws got the machine running and cutting quite well, but the idle speed is way too fast. I can't seem to slow down the idle without also killing the cutting speed and power.

 

Can anyone offer any advice on tuning a carb of this type?

 

Sadly I have little knowledge of the history or age of the machine, or whether in the past it's had replacement parts fitted. Genearlly it seems to be little used and in pretty good condition, certainly too good just to chuck away. My intention if I can get it running properly is to keep it as a spare machine.

 

Thanks.

I think from memory the Torx takes up slack on the throttle cable, so this would increase idle speed if screwed in too far. The other screw maybe a low speed adjustment. If so set this screw to get the fastest idle then slow the idle with the torx screw. Please post a picture as it has been sometime since I have worked on these.

 

Hope I have been of some help.

[spudulike]

From memory, take up the slack in the throttle cable with the torx screw but not to the point where it changes the idle speed and then use the screw to get an even idle speed - the carb has fixed jets and if it is like the MS170 carb, has a single screw that adjusts air input for the correct idle speed.

[Treewolf]

Thanks guys!

 

I'll try to get some photos but it won't be till next weekend now.

 

What you say makes sense though, since to start with the torx screw was rattling about loose, and although tickover was OK the engine simply wouldn't pick up beyond about half speed (and had no power, but that is probably to be expected at half speed). After I had screwed the torx screw in, with the throttle open, to get a decent running speed, then the idle is way to fast.

 

I didn't realise that it was a fixed jet carb, but in view of this I will investigate further.

 

Thanks again!

[Rich2484]

I would clean the carb out fully and the fuel filter. Not knowing what past the machine has. Could be gunked up with crappy fuel or crud.