Stihl gas trimmer dies with throttle - even with new carb!

Stihl FS gas trimmer bogs down on throttle - even with brand new carb. Turns out the piston / cylinder is scored. I would have spent hours fiddling with the carb if I hadn’t had seen this issue.

So, a trimmer with lower compression due to a scored piston can mimic the same behaviour of an out of tune carb.

Good to know!

[mikepicard]

Thanks for this, I too tried a new carburetor, new fuel filter, and cleaning the spark arrestor but I think mine is a goner too

[alexcarr]

What you are describing is the same thing happening to my FC 110. 4 stroke. I adjusted the valves, new cam gear, exhaust is not plugged. Only has hi test gas with a good mix oil. I have owned this machine for more that 10 years. When it started acting bad I could start it and let it idle a few minutes and it would clear up and run perfect. Eventually the time of idle increased to 10 minutes and now I am trying to solve the issue. The issue is it boggs out from idle and will not accept any throttle. But after a long idle time it just clears up. Not gradually but suddenly. And works perfect for as long as I run it. Run out of fuel, fill it up and it works perfectly. Put it away. Next day, won't Rev up on start-up. What do you think? I thought the decompression tab on the cam gear was sticking in, decompression mode. It's not that. With the new cam gear it's not that. It must be the carb. Adjusting the H and L has no effect on the issue.

[anvilhead]

Worth checking the exhaust port and muffler too, make sure they’re not blocked.

[mada]

Thankyou sir.my oil was dirty and there was hardly any.trimmer fixed.

[sharon]

Where do I put the oil and what kind???. Its an SF 94R got it Brand
new in 2016?
Is been bogging down etc, ehat anout the piston and air filter, ? Could that be it also?

[robbarnes]

Isn't that a 2 cycle engine? If so then what did you mean when you said it was run without oil? Do you think someone ran straight gas in it? There isn't anywhere that we need to add oil is there?

[lukebilliot]

Bogging down is normally the spark arrester. U can take it off and see if it’s clogged.

[cousinskeeter]

New carburetor??? I just bought mine only used it twice and this is happening.

[reyalPRON]

could you not tell by the lack of resistance when pulling the cord? thats how i "measure" comp :)

[footprint_]

I have a John Deere XT105 weed whip. This motor looks just like mine, save for the Stihl plastic.

One fine day, the one and only bolt that holds the stator to the crankshaft decided to loosen, creating a no start condition and ruining the timing. Ran like a champ before this happened, so I know I only have a timing problem.

I got everything as close to TDC as I could by eye. It starts and idles now - certainly an improvement, but when you give it gas it just bogs and dies. There are no obvious timing marks, however the crankshaft is female notched. I have a hunch that the stator had a male notch for precision placement on the crankshaft. The notch on the stator broke off when the nut came loose, ruining the timing placement.

My point - if everything is good/new/replaced and you’re still having trouble with your small engine reaching maximum power band, timing is suspect.

Hope this helps somebody!

[aust]

If your engine has good compression why does it happen

[winsmith]

That looks like ur piston rings are ceased. so no compression.

[leadnsteel]

Yeah usually trimmers only last a few years before they need a rebuild.
You might as well throw it away