Stihl FS55R Trimmer wont run under load

andy_levineJune 17, 2010

I have a Stihl FS55R string trimmer. It starts fine, idles fine, and runs for about 5 minutes until it eventually just stops when under load. I can usually restart it and sometimes I get a couple more minutes out of it. The unit is less than 2 years old, was stored over the winter empty, is using fresh fuel with Stihl Oil in it.

I found a message on this forum with someone with identical problems, so I tried several of the solutions. So far I have:

1) Removed and cleaned the spark arrestor. Originally I cleaned with carb cleaner and a brass brush. When that barely helped, I pulled it again and burned everything out with a propane torch. The spark arrestor is clean

2) Tried running totally without spark arrestor

3) Replaced spark plug

4) Replaced fuel filter

5) Tried adjusting fuel mix until the feed head barely started turning

6) Tried running with fuel cap off

The unit still does not function properly. The only other thing I can think of doing is to rebuild the carb but I dont have the parts, the know-how or the time. My local Stihl dealer cant get to it for almost a month.

Any help GREATLY appreciated!!



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You take the muffler off the make sure the exhaust port is not clogged?

After checking that, mix 2oz of SeaFoam to one gallon of gas, run a tankful to clean the carb and see whether that will help.

Write back after trying these.

    Bookmark   June 18, 2010 at 1:48AM
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masiman(z7 VA)

Try getting a fresh batch of gas again. Sometimes even a "fresh" from the pump batch is marginal.

It won't be the exhaust port if he is able to run it for 5 minutes before it starts to die (I assume it runs under load in those first five minutes). If exhaust is clogged he would not be able to run load at all if it started and it would be obvious from the get go, not after five minutes. I would guess a fuel problem or an air leak. Chances are your spark is working but you can confirm that if it is checked shortly after it stops running.

I would compression test (probably good since you are getting it started). More importantly I'd do a pressure/vac test. I'd also do a good carb rebuild. The parts are easy to get and not too expensive. They are really not that difficult. But I understand if you don't have time or the desire to do that. A pressure/vac test is more involved in terms of tools. The only other option I could suggest would be to find a local guy that is good with small engines.

    Bookmark   June 18, 2010 at 4:50PM
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Yungman....exhaust didnt seemed blocked. I couldnt find SeaFoam locally but I tried another ether based fuel line cleaner (Start Your Engine) that appears to be similar. It ran fine for about 5 minutes, the conked out again. THen it wouldnt even idle well after that. I've messed around with the fuel mixture screws and the low altitude screws so much now Im not sure how to get them "correct". They are marked as 1 full turn on the low speed screw and 3/4 turn on high speed. The Low Altitude screw is adjusted until the head doesnt turn.

masiman...I found someone locally who said he could look at it tomorrow. Dont know if he knows his stuff, but hopefully so.

Frustrating for a pretty new machine....I found the receipt when I was looking for the manual and I bought it last May so it is barely 13 months old. This is the 2nd Stihl tool that has given me problems. They had to replace the clutch in my chainsaw after 3 months. Just waiting for my blower to die next....:(

    Bookmark   June 18, 2010 at 11:02PM
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Don't rule out carb issue, My Honda behave like that, it ran find until a few minutes into trimming then it started to bog when applying throttle. It was a blocked carb. I actually found metal fragments that plug the jets. I cleared it and it work.

SeaFoam is good for cleaning carb. You have to run a tankful of the mix to tell the difference. I hate to have you go open the carb at this point.

As for vacuum leak, you can always spray a little soap water at the seams and run the engine to see any bubbles come out. For a one year old trimmer, my bet is dirty carb. Even if you empty your tank, there are still gas in the carb and the primer bulb. they do go bad. best is to use Stabil. When I put on long term storage, after empty the tank, I position the primer bulb to face up and prime out all the gas. re-empty the tank again. then I close the choke, hold the throttle WOT and start it to run it dry.

I won't spend any money until I run a tankful of SeaFoam mix. Takes one tank to clean it. It is not going to do magic.

    Bookmark   June 19, 2010 at 3:33AM
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And the winner is.... DIRTY CARB!!!! ok, no surpise I guess. He tore it down while I was there, cleaned everything, blew it out. Found a bit of sludge in a small screen fuel filter nside the carb body and figured that was it. Took it home and ran it for almost 3.5 hours yesterday.

I guess I've learned my lesson about refeuling around clippings and the TRUE definition of "fresh fuel"

    Bookmark   June 20, 2010 at 6:22PM
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Glad you fixed it. Clipping usually don't get into the carb because it is too big to pass through the gas filter. It is either inside originally and work it's way to the critical passage like in my case or it is gum formed from stale gas. Use Stabil and try my way of winterizing next time.

    Bookmark   June 20, 2010 at 6:42PM
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Junk it and get an Echo or Tanaka.

    Bookmark   June 21, 2010 at 9:11AM
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Your better off treating a 2.5 gallon batch of fuel with Stabil and using it.
These machines run so little each period, then sit over winter.
Leave the treated fuel in the machine to keep the parts wetted.
These carbs have diaphragms that control fuel flow, so wet is better.
Store the machine in a natural run position

A handy computer app is a calendar that pops up once every 3 weeks to run the machine. There just is not a better process.
It burns off moisture with occurs naturally, that can damage bearings.

W/O Stabil, the fuel separates creating crystals that harden like rock blocking ports.

As for the suggestion by gorper99, those machines don't stand a chance sitting next to any other under the same conditions.
Not very helpful.

    Bookmark   September 12, 2010 at 9:59PM
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