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Hello,

I’m sure the answer for this is somewhere on this forum, but I’ve searched through tons of them and feel like I’ve tried everything, and still can’t seem to figure out the source of the problem. I know each trimmer’s problem can be slightly unique, so thought I’d just post what I’ve done and see if someone can crack the code;

Trimmer - Stihl FS55R

Original Problem: Trimmer bogging down badly, stuttering and dying when trying to idle.

Original Solution:
1) Emptied Gas
2) Replaced Carbeurator (cleaned area well)
3) Replaced Fuel Lines and Fuel Filter
4) Replaced Gaskets (pin hole in correct place)
5) Put in fresh fuel/oil with Sta-Bil
6) Replaced Spark Plug
7) Checked Muffler/Spark Arrestor

Pumped the primer and fired up the trimmer and it started on 2 pulls, ran top speed like a dream, extremely smooth - Felt like it was all back to normal.

BUT, now the SECOND I let off the trigger (full idle) it dies *Immediately*. Doesn’t sputter to a stop, I mean completely dies. Will not even rough idle, just completely dies.



Since then, have tried:

- Adjusting carbeurator (both L and LA) - no impact

-Taking the carbeurator back off, verified everything was clear, pin hole was in correct place on gasket - new carbeurator’s pin hole lined up with hole on gasket - no blockage.

- Tried running with fuel cap off to see if airflow was problem. - No impact

What am I missing? If it starts up and runs like a dream, but will absolutely die instantly the second the trigger is let off? My thoughts go to airflow or fuel/ fuel flow on idle, but like I said I replaced fuel filter and lines… Spark arrestor is clear, put in fresh fuel, added fresh air filter, and verified the carb’s pin hole is clear when the carb is closed.

I’m at a loss.

First is OEM carburetor or after market? Even OEM carburetors can have the metering set too low from the factory but aftermarket carb are hit and miss as if they are good or not.

On initial set mixture screw to 1-1/2 CCW from fully seated position. Yes Stihl has different initial settings but this is the will start settings as they are constantly subbing carburetors. You will need a tach to make these adjustments. Also make the plug gap is set correctly as noted below.


There are times when you got to start with LA all the way in to start adjusting things.

Why don't you raise the idle speed a bunch until it doesn't die anymore, and see where we are after that? It would give us a better idea.

With a situation like yours the first thing I do is a leak down test & pressure test of the crankcase
Note it has the be both pressure & vaccuum .
leaking crankcase seals can cause these symptoms .

If the pressure tests are OK then the nest trick I use is to replace the idle screw with one long enough to hold the throttle WFO
Start the trimer and screw the throttle stop as far in as the engine will continue to run
Screw the L jet fully home so you are only running on the H needle
Turn the needle in till the engine faulters then out till the engine faluters then return it to slightly further out than 1/2 way between the two .
Back off the throttle stop screw till then engine starts to faulter and open then L needle till it just starts to runs clean .
Continue along this pattern till the throttle is at the proper position and the clutch no longer engages .

Done this way the H will end up slightly rich and may need to be turned in a little .
Doing it by ear is a lot harder now days because the difference between too rich & too lean can be better than 2 turns rather than < 1 turn as was the case with old carbs.

Also note running too lean for too long can & will damage your engine so don't take all day .
Jets & jet holes are easily damaged and this is worse with the new carbs as they come together very slowly rather than with an abrupt stop .

Also check the secondary compression
If it has not got better than 90 PSI then you will be pushing it to ever run properly

From your first post appears changing the carb did not affect the idle problem. If you installed an OEM carb with the mixture screws set to the correct initial settings per the chart Star provided and it won't idle here are things to check.
Remove the muffler and check piston and cylinder for any scoring. Doesn't matter what pulling the rope feels like. Remove the muffler and check. Like Bert said it should be pressure and vac tested but most folks don't have the tools for that. If there is any scoring that can cause the poor idle problem. If there is no scoring then most likely a leaking crank seal. You can diassemble the unit to see the seals and see if there is any oil on them. If there is oil seeping from a seal it is not holding pressure.

As said just pulling the rope many inexperience techs think they got plenty of compression. Well here I use a compression gauge and I know that most 2 cycle hand held equipment requires at least 100 psi to operate correctly. At that what my reads normally and I only seen a couple that would below 100 psi.

Even though a two cycle is one of the simplest engines around they are very picky about things being right. As crankcase seals (and this includes gaskets) must be leak proof.

I would recommend taking the unit to a good two cycle repair shop as they got both the equipment and experience to deal with these machines. Just throwing parts just don't cut it and does get expensive. Now of course I am self trained on these by buying and repairing a whole truck load of what was considered junk hand held equipment from another shop.

Now this FS55 starting it sounds like it just mal-adjusted carburetor wise. I have seen units that have to be initially set with the idle speed (LA) set in all the way just to start tuning an engine. I also have seen where metering levers were set so low that the engine would starve for fuel at idle (even new carbs).

You mention you checked the spark arrestor...could you see through the screen? Is your machine low hours? I have a Combi that was acting up last fall and would be very slow to accelerate. This spring I couldn't get it to run above idle, couldn't actually get it off choke. It was a clogged spark arrester. Mine is a barrel shape that has a hex on the end and threads into the muffler. I couldn't see through it when I removed it. I heated the screen with a MAPP gas torch and it glowed orange. After it cooled, I tapped it and blew the soot off with compressed air and my machine started and ran great. Just something easy to try before you go to great lengths in other areas.