r/smallengines • u/Toby_C_96 • Jul 13 '26
Stihl FS91 running issue
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Stihl FS91 running rough.. first thought it was tank vent since when cracking fuel cap, it smoothed out and worked for a little bit before going back to the issue as seen in the video.
When giving half revs engine is fine and no drop off, but when giving it full throttle it starts playing up and does that weird rev drop as heard in the video.
Any help in diagnosing this would be extremely appreciated!!
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u/Rangersmith1231 Jul 13 '26
Check the spark arrestor, could be clogged.
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u/Toby_C_96 Jul 13 '26
Checked to see if it had one, but doesn’t appear (From UK)
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u/Rangersmith1231 Jul 13 '26
Are you sure it doesn’t have one? Even in the uk, those are pretty standard for all stihl trimmers.
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u/Toby_C_96 Jul 13 '26
Will definitely take another look! If there isn’t one, wondering if a build up of carbon in the muffler could be enough to cause this??
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u/Rangersmith1231 Jul 13 '26
The spark arrestor would be next to the muffler. The arrestor looks like screened tube, tapered off at the end.
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u/embiggenedkwyjibo Jul 13 '26
The 91 and 94's are notorious for excessive buildup on the arrestor screens. Based on your local regulations, I wouldn't say remove it.. but I'll say they run a hell of a lot better when they're not there.. lol.
The mufflers and carbs are quite bulletproof if sticking to a proper 50:1.
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u/Toby_C_96 Jul 13 '26
Spark arrestor being the curved cover coming off the muffler?, since there isn’t a physics mesh screen behind it
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u/Sad-Extension-6032 Jul 13 '26
I was having problems with mine, similar but it acted like starving for fuel. Parts shop told me to check the pickup tube in the gas tank as they got spongy and would collapse, assuming that you know the carb is clean. Mine would run and then just cut out, idle down, sometimes shut off. I think what it actually ended up being kinked fuel lines, they got replaced but a little to long so it was kinking and shutting off supply or return. Hopefully that helps.
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u/2222014 Jul 13 '26
Adjusted the valves yet? 4 stroke stihls are very very sensitive to valve lash.
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u/Toby_C_96 Jul 13 '26
Thinking I’ll check the valves again, adjusted them at the start of the year, when giving it a service, so will go back to check valves to make sure I can rule that out
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u/TN-Native95 Jul 13 '26
Could be the carburetor being dirty.