Stihl FS91 brushcutter

[Andrew]

Morning all, i have just joined the forum hoping to get some ideas about a fault on my Stihl FS91 brushcutter.  I've been using it quite a lot recently with a bramble blade, so it's been working quite hard, at full throttle most of the time.  A few weeks ago, it developed a strange fault.  While on full throttle, it would slow down as if the throttle trigger had been released, then speed up again. It would idle fine, and start easily.  But it wouldn't keep running at max revs, it kept slowing down and speeding up.  I took it to a local repairer and he replaced the black vent valve in the top of the fuel tank which is connected by a rubber tube to the air filter, and he though this valve was blocked and so wouldn't allow fuel to be drawn from the tank (as the tank would now be airtight).  That fixed it, or at least it did for a a while.
 
I used it again for half a day with no probs, but the next day after a couple of hours it started doing the same thing again.

I have ordered a couple more genuine stihl spare vent valves, as they are not difficult to change.  I sent a video (with sound so it was obvious that it was slowing down and speeding up) to enquiries at Stihl UK but was just told to take it to my local stihl dealer. I'd rather not pay loads to get it fixed.

Has anyone else had this problem, and if so, was it caused by this vent valve?

[Andrew]

I don't know how to add a link for the video i took, it's 120Mb so i can't upload it.  Sorry, even worse at IT than fixing brushcutters 

[Mick]

Hi Andrew, and welcome to TGM-Forum.

Not one of my favourite engines, they do suffer with strange symptoms caused by various issues.  Make sure fuel line is good and not gone soft like putty, and make sure the fuel filter is clean and clear of any restriction.

Also make sure the valve cover hasn't cracked or leaking around the gasket and copper washer otherwise it reduces the efficiency of the pulse for the fuel pump. And the short pulse hose from crankcase to bottom of carb, make sure that is a good fit also.

Failing that, a carb clean might be required.

Regarding your video, where are you trying to link it from, Youtube?

[Andrew]

Well, i tried putting it on YouTube but i go straight to the channel for the Carmarthenshire Meadows Group as i created that channel, so anything i try to send from my email address to youtube goes on there, and i didn;t think i should put my brushcutter malfunction video on there!  It's on my hard drive, i'm not sure what else i can do with it 

[Mick]

It's on my hard drive, i'm not sure what else i can do with it 

Unfortunately this site can't handle large video files, they can be embedded or linked to depending on where they are hosted. Images are not an issue but videos just take up far too much server space.

You could try one of the social media platforms, Facebook or Instagram etc and post a link to it here.  Or start another temp channel on youtube.

[Andrew]

Never mind, it was a rather boring video actually. But here's a photo of the fuel tank vent.



I had another go with the machine this morning. I was using it to do hard work with bramble blade again on head high brambles so full throttle operation. To test the theory that the slowing down/speeding up is caused by fuel starvation due to the tank vent no letting air in quickly enough, when the symptoms started (a couple of munutes after starting cutting) I just let the engine return to idle, loosened the fuel filler cap, and did it up again. The symptms disappeared, and it ran fine for another couple of minutes, then exactly the same thing happened again. 5 times (until i'd finished the job) the fault was cured for a few minutes by loosenening the fuel cap momentarily, then the symnptoms returned. I think that's quite good evidence that the tank vent isn't letting air in quickly enough.

When the local repairer (All Small Engines, in Manordeilo, Carmarthenshire) replaced the vent the machine was fine for a dy nd a half, then the symptoms returned. I have been using the fancy Aspen 2-stroke fuel mix in the brushcutter, and i wonder if some component in the fuel could be reacting with one of the parts of the vent? I think i'll go back to garage petrol + Stihl 2 stroke oil with fuel stabiliser.

[Mick]

That's a strange one Andrew.  You should be able to test the vent by blowing into the pipe fitting at the top. It should let air in but not out.  The alkylate fuel can make rubber parts go hard if they have been running pump fuel for some time, but I'm assuming this was a brand new vent.

[Andrew]

Well, I've ordered a couple of them and made sure they were genuine Stihl ones. I'll try replacing it and see what happens. But i must say, i'm not very impressed with the reliability of this machine. I bought it because i was so impressed with my Stihl chainsaw, and  2ndhand long reach hedge trimmer which I hve had for longer, and they''ve both been totally reliable (touch veneered chipboard of my desk  ), but this brushcutter has gone wrong several times now, and i think i agree with you that the 4-mix engine is maybe too complicated for its own good. I'm tempted to sell it and get something else, like a real 4-stroke Honda brushcutter. I had a 2ndhand Honda 250 motorbike for about 4 years and it NEVER went wrong.

[Mick]

Yes the 4-mix has it's issues, carbs, valve guides, plastic camshafts where the metal parts of the decompressor breaks apart etc. They also like running on the green HP Super oil (Semi-synthetic oil) instead of the red stuff (Mineral oil).