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#115196 01/05/22 08:09 AM
Joined: Aug 2021
Posts: 9
Likes: 1
RodP Offline OP
My FS90 brushcutter loses power after about 15-20 min. Lots of power beforehand, and idles well any time.
Has a new spark plug, new air filter, new std fuel with Stihl oil. One bloke said it likely the coil.
Any suggestions on what else may be an issue.

Joined: Oct 2020
Posts: 125
Likes: 14
Apprentice level 2
Hi Rod

Have you checked the spark arrestor

Page 36 and 37 of the Stihl manual

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pge 37.png (168.13 KB, 38 downloads)

Only hurts till the pain's gone.
1 member likes this: speedy
Joined: Jul 2018
Posts: 2,090
Likes: 81
As Nath said, spark arrestor is a possible culprit, as is the coil - though it will often just die after getting hot when its a coil

As I am pretty sure the fs90 is a 4mix stihl, I would be tearing in and checking the valve clearances.

For those readers who are unaware of the stihl 4mix, it is a four stroke engine that still requires 2 stroke mix

Shindaiwa do a similar one

Joined: Apr 2014
Posts: 219
Likes: 10
Apprentice level 3
Tyler has probably dealt with way more of these than me and that sounds dead right.

The issue with the arrestor tends to show up from first start with the motor feeling choked and not wanting to rev up. So, if it starts and runs freely from the outset, it’s less likely. However, I’ve seen symptoms similar to those you describe earlier on before the arrestor is more blocked. It’d be worthwhile taking the arrestor out and burning any residue off as that’s quick and simple. I sit them on a fire brick with another behind and apply a light oxidising flame from a map torch until there’s no soot remaining.

The valve strip down isn’t overly complex and there are instructions readily available from Stihl. These engines don’t like extended part-throttle usage, especially when run using the stihl 2-stroke oil. Not bagging the oil, I use it myself, but I find it needs a good proportion of the operation to be in the upper part of the rev range or things do gum up.

I’ve also seen the combustion chambers pretty filthy on machines that haven’t been given much full throttle use and, in one situation, gummed up rings. It’s not a difficult thing to fix, provided that the problem’s been caught early on. It can just require a gasket kit, a strip down, thorough clean and reassembly.

What tends to happen is the partially burned combustion products coat the surfaces. So, when the engine heats up and parts expand, some of the designed clearances are not available and the engine tightens up and won’t rev.

The higher performance 2-stroke oils work exceptionally well at providing lubrication at higher temperatures and duty cycles. However, if the engines don’t reach the designed temperatures sufficiently frequently you can end up with a sooty and sometimes tarry residue that prevents normal operation.

Joined: Jul 2018
Posts: 2,090
Likes: 81
Hi Ironbark

Haven't done much with these motors as I hate them with a vengeance after fighting one to start and ripping my finger open on a poorly finished plastic guard.

The stihl range has fallen significantly in my opinion - fs38 being among the worst trimmer I have ever used.

My theory is if you are going to make a 4 stroke, use a sump with oil - don't stihl (pun intended) need users to mix oil. Also don't like plastic cam gears on commercial machines running 8000+rpm. These are known for cam wear issues when run hard in thick grass. That stuffs with the valve lash and everything.

Only one lob controlling both valves too so 2x the wear.

They do go well though - would love to compare one side by side with my old FS81 - amazing machine

As much as I don't personally like honda gx25 due to weight, I can see their benefits. I still have a ryobi 4 stroke trimmer from the late 90s and it is really nice to use because its light, easy start and has enough power but no top end.

100% agree on the stihl oil - hates part throttle operation. I have a bottle of stihl ultra I bought for $5 when stratco cleared out stihl. I won't use it as a lot of my use is part throttle.

Joined: Aug 2021
Posts: 9
Likes: 1
RodP Offline OP
Many thanks for all the obvious knowledgeable advice.
The timing and settings of the valves have been done, but the problem still exists.
The spark arrester was not thought of, so I will look at that as well as the coil, and hopefully one or both will be be the culprit(s).
Watch this space.

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