Help with Stihl FS85 Trimmer

tillboy2001 · Apr 22, 2008

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?

westbrooklawn · Apr 22, 2008

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.

tillboy2001 · Apr 22, 2008

Thanks. I guess I'll give the carb another cleaning and verify the fuel line/filter.

Flatheadyoungin · Apr 22, 2008

does your's have the littls plastic stops on the H and L speed adjustments? if so, take them off and you can adjust it as needed....

it's the little black plastic caps on the H and L adjustments, and they will only allow you to turn the screws so far......very limiting

tillboy2001 · Apr 22, 2008

Yes it has those. I tweaked those but it made no difference. I suspect something is still clogged in the fuel system somewhere...I'll keep working on it.

Flatheadyoungin · Apr 22, 2008

i'll put my money on the collapsed fuel line or clogged filter.

i know you said you cleaned it but there ususally is a small filter screen in the carb too......sometimes, i just remove it with a pocket knife.....

pz31g22 said:
Yes it has those. I tweaked those but it made no difference. I suspect something is still clogged in the fuel system somewhere...I'll keep working on it.

afternoonhandyman · Aug 29, 2009

I'm having the same problem with my Dad's FS-85. It has the Zama carb with 1 (one) adjusting screw on the carb, which is not a needle, just flat-ended with smooth shaft for about 1/4" before the threads. There is a 2nd on the throttle cable cam. Can someone provide me with adjustment procedure on these? I've spent 6 hours Googleing and ASKing and can only find info on the 2-screw carbs. The manual thaqt came with the '85 shows the 2-screw carb. No hidden or covered screws either.

Thanks in advance

PILOON · Aug 30, 2009

Most of these small 2 strokes (saws trimmers etc) have a flopping gas pickup in the tank.
Generally a soft rubber line with weighted end that contains a felt filter.
If the filter gets clogged that will limit the fuel flow and even water can clog it.

Just a thought that might help. Good luck.

Bird · Aug 30, 2009

PILOON said:
Most of these small 2 strokes (saws trimmers etc) have a flopping gas pickup in the tank.
Generally a soft rubber line with weighted end that contains a felt filter.
If the filter gets clogged that will limit the fuel flow and even water can clog it.

Just a thought that might help. Good luck.

I ran into that little problem with my Mantis tiller; easily corrected after I learned what the problem was. And also with the Mantis, I had a similar problem as originally reported in this thread. It would start and idle just fine and the instant I tried to give it gas, it would die. That one took me awhile to find and correct, but it was actually nothing more than a simple carburetor adjustment.

But I also learned with the Mantis that you must allow it to idle and warm up two to three minutes before using the throttle or it'll die. My Stihl string trimmer has never had that problem.

dennis1960 · Jul 2, 2011

Stumbled on this site while searching for parts. I just had the same issue with a Stihl FS80. Pull the muffler off and you will probably find the exhaust port is plugged.

Jeff Lary · Jul 2, 2011

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

tdklex · Jul 11, 2011

westbrooklawn said:
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 · Jul 15, 2011

New lines?? Mabe new carb/rebuild possibly.

tdklex · Jul 15, 2011

Johndeere50d said:
New lines?? Mabe new carb/rebuild possibly.

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

Johndeere50d · Jul 16, 2011

tdklex said:
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 · Jul 16, 2011

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 · Jul 19, 2011

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 · Jul 20, 2011

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 · Aug 10, 2011

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 · Aug 19, 2011

pz31g22 said:
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>