Help with Stihl FS85 Trimmer

[Jeff Lary]

This may sound nuts but check the exhaust port hole a mud dauber filled mine,.... acted the same way.

 

[tdklex]

Sounds like a fuel problem to me. Could be that carb is still dirty, filter on intake line plugged, or deteriorated fuel lines (a very common problem which is sometimes hard to see). Sometimes when my 2 cycle equipment starts to act up I run a little Sea Foam in with the gas.... it is amazing what this stuff will do. I'd try that first, and also replace the fuel and vent lines if they are more than a couple years old. Oh... also make sure your air filter is not plugged.

I have same problem. can you tell me how to access the carb to adjust?

 

[Johndeere50d]

New lines?? Mabe new carb/rebuild possibly.

 

[tdklex]

New lines?? Mabe new carb/rebuild possibly.

Thanks. can you tell me approx. how much a rebuild or new carb. would be?

 

[Johndeere50d]

Thanks. can you tell me approx. how much a rebuild or new carb. would be?

Yeah, 100$ for a new carb, but you can get A rebuild kit for 25$

 

[Chilly807]

Most small gas engines have a screen for a spark arrestor over the muffler outlet. Over time, the carbon buildup on the screen will choke off the exhaust flow and the engine won't rev freely.

If your's has the screen, I'd try removing it and see if that helps. If it does, either get a new screen or gently wire brush the old one.

I had the same problem with a Jonsered chain saw a number of years ago.

As someone else said, it's possible the exhaust is choked with carbon further back.

Did this happen all of a sudden, or get gradually worse over a few days of use?

My saw worked fine one morning, by the end of the day it wouldn't go past an idle. Started first pull, idled fine, then wouldn't rev at all.

Sean

 

[CWOSCSU]

Had a similar problem with a Sthil KM85 (same engine as the FS85). It would idle Ok on the ground, but I'd pick it up and it would run like crap and then die. Turns out I had somehow twisted the fuel line when I replaced the filter, and the filter was up towards the top of the tank. Was sucking air when I picked it up. A simple twist of the fuel line fixed it.

 

[niemeyjt]

I see OP was April. Was it first time this year you used strimmer after winter? Did you drain it of all fuel last autumn? One problem with the newer unleaded ULS fuels is that they can gum up carbs if left over winter.

Also, check fuel pipe from tank to carb has not split.

J

 

[Sprocket]

I own several pieces of Stihl equipment, all of them are very picky about stale fuel. If you are running gas that has been in a gas can for more than a month I would suggest trying some fresh 89 octane or higher. I was going crazy with my chain saw and Trimmer cleaning carbs and changing plugs until I found out it was just some stale fuel. I verified this by trying it in my other Stihl trimmer and it developed the same symptoms (poor running above idle) A little sea foam and fresh mid grade gas and all is well again.

I hope this helps somebody.

Edit; The new Stihl oil has a fuel stabilizer in it to help with this known issue and the newer owners manuals mention it as well.

 

[jh60]

I've got a FS85 trimmer from Stihl. It will run just above idle but dies if you try to give it more throttle. I've taken the carb off and taken it apart to blow it out. Also replaced the coil/spark plug wire since the plug wire was coming loose on the end that connects to the plug. I can't figure out why it won't run right, as in fast. Like I said, it'll only run just above idle. It sounds as if it's starving for fuel. I checked the fuel filter and it looks ok.

I noticed on the new coil, there is some adjustment there. Meaning it can be very close of farther away from the flywheel. I didn't see any specs in the book on what distance to place the coil from the flywheel....but I don't know if that has anything to do with my problem or not. It ran this way before I replaced the coil. This trimmer actually belongs to an in-law - I'm trying to fix it for him. In the past I've been able to blow out the carb to fix something like this. I can't imagine it being anything other than the carb or coil...there's not much else to this simple machine. I'm still leaning towards the carb. Maybe I didn't get it cleaned out well enough? Who knows.

Any ideas?

I am having the same problem with mine. I just joined and posted a question about this before I ran across yours. OOPS>