Stihl brushcutter problems

[Sprint]

Hi guys

Recently my ~6yo Stihl FS90z brushcutter has been bogging down even in light grass, and holding the throttle flat after the first 20 minutes or so basically causes it to die right off and stop.

I've given it fresh air and fuel filters, new spark plug and a fresh fuel mix, but its still doing it, although 10 minutes of pulsing the throttle the last time I ran it seemed to have temporarily cured its woes, but no idea if its going to do it again....

any ideas?

[Homestar]

Sounds like the mixture has changed slightly and may be running a tad rich. You should find 2 screws on the carby - marked hi and lo. I'd start by screwing in the hi screw a touch while holding the throttle open to see if it responds better. There are some good on line tutorials on how to tune a 2 stroke engine. Ill see if I can find one I used last time - it worked for me on an old chainsaw I was playing with.

[Eevo]

i concur, play with the high needle a bit.
good luck

[Sprint]

Tried playing with the mixtures, they were wound right up and didnt help the situation when I turned them down....

[Alex 110]

I agree with it being the mixtures, but I found it virtually impossible to tune my stihl km90r & ended up taking to an expert. Well worth the expense IMHO

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[AllTerr]

Try taking the muffler off and checking for clogs. Then look at the exhaust port on the motor. Sometimes they get gummed up with soot and will cause all sorts of bogging.....


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[robbotd5]

Does it have the one piece fuel delivery/return black rubber tubing?.
I had the same issues with mine. Turned out to be a perished fuel delivery tube making the mixture too lean. I ended up just using some aquarium air tubing to replace the inlet/return and have not had an issue since. I was a little dubious about the compatibility of the tubing with fuel but so far, all good.
Regards
Robbo

[bee utey]

The one way air inlet valve on the filler cap may be too tight, rubber gone a bit hard. Unscrew the cap for a second while its bogging down and see if it picks up. I had this problem with a new brush cutter recently, I stuck a sharp knife in the valve and reduced its tightness a bit. Works perfectly now.

[Don 130]

mud wasps got into the muffler on mine and made it bog down and hard to start.
Don.

[slug_burner]

mud wasps got into the muffler on mine and made it bog down and hard to start.
Don.

I have found the same on several chainsaws and trimmers.

[mick88]

Before you play with the carby screws drain the fuel out and mix up some new fresh fuel to the exact ratio. Most small engine two stroke issues are with fuel going stale or incorrect mix. Not mixture/air screws altering. Make sure the nylon is not too long as that will make it labour. Also check the exhaust for any carbon build up and burn it out if necessary.
Good luck.


Cheers, Mick.

[Sprint]

Before you play with the carby screws drain the fuel out and mix up some new fresh fuel to the exact ratio. Most small engine two stroke issues are with fuel going stale or incorrect mix. Not mixture/air screws altering. Make sure the nylon is not too long as that will make it labour. Also check the exhaust for any carbon build up and burn it out if necessary.
Good luck.


Cheers, Mick.

Its got fresh fuel in it, only a couple of days old....


Try taking the muffler off and checking for clogs. Then look at the exhaust port on the motor. Sometimes they get gummed up with soot and will cause all sorts of bogging.....


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a little bit of soot, but clear


Does it have the one piece fuel delivery/return black rubber tubing?.
I had the same issues with mine. Turned out to be a perished fuel delivery tube making the mixture too lean. I ended up just using some aquarium air tubing to replace the inlet/return and have not had an issue since. I was a little dubious about the compatibility of the tubing with fuel but so far, all good.
Regards
Robbo

its got 3 seperate hoses running to the tank


The one way air inlet valve on the filler cap may be too tight, rubber gone a bit hard. Unscrew the cap for a second while its bogging down and see if it picks up. I had this problem with a new brush cutter recently, I stuck a sharp knife in the valve and reduced its tightness a bit. Works perfectly now.

I'll try popping the cap next time it plays up

[jboot51]

Check all the crankcase bolts are tight.

[ian4002000]

My stihl brushcutter was giving trouble a while ago due to a blocked muffler.
Take the exhaust off, soak it in petrol and then light it to burn off the excess oil inside it. If this makes no effect you then probably have a carby problem with dirt in a jet somewhere.

[460cixy]

How old is it? My money is on carb diaphragms and or leaky crank seal fuel hoses should be supple not hard and brittle or rotton

[Sprint]

fuel hoses are fine, and same for the muffler

[460cixy]

Time for carb kit and check for air leaks I think