# Stihl FS-80 trimmer runs for 15 minutes



## Captgerryf

Back on the forums again with my Stihl FS-80 trimmer. Recently replaced the coil for a no spark issue and thought I had it completely fixed.

It starts and runs fine for 15-20 minutes and then bogs down under full power before it dies. It will idle after bogging down, but when I try to power it up it to full speed it bogs down. If I don't let off of the throttle and go back to idle, it will die out. I then have to wait till it cools down to restart. 

I've seen other posts that the coil is bad and causes these symptoms, but as I said, I just replaced it with a genuine Stihl coil. Also seen where a clogged spark arrestor might cause this. I removed the spark arrestor before I used it yesterday and it still bogged down at the 15-20 minute mark.

I pulled up the FS-80 manual from Stihl and it shows a low speed and high speed carb screw, but my carb only has a low speed screw and an idle speed screw. I have it at one turn open. I think it is a Walbro, but not 100% sure. I can look for the exact model # of carb if needed.

What should I be checking to solve this issue. I cleaned th carb at the beginning of this year while I was doing the coil replacement. Also there is an excessive amount of black oily residue at the muffler.

I need the trimmer to run more than 15-20 minutes because my property can't be trimmed in that amount of time.

Any help is appreciated.


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## glenjudy

Personally, I don't see a problem here, after 15-20 minutes of weed eatin', I'm ready to sit down and rest anyway...(jus' kidding)

what 2 cycle oil are you using? and at what ratio are you mixing with gas?

when you cleaned the carb, did you pay particular attention to the metering lever height?

Does it quickly accelerate when you blip the throttle trigger?

thanks,


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## geogrubb

Sounds like the fuel cap is not venting properly, try running with the cap loose to see what happens. Have a good one. Geo


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## TnShooter83

geogrubb said:


> Sounds like the fuel cap is not venting properly, try running with the cap loose to see what happens. Have a good one. Geo


That is what I'm thinking also.


I had a Leaf blower once with the EXACT same symptoms as you described above. Turned out be a clogged fuel cap vent. 

Good luck.....


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## rotti1968

The oily black residue is unburned oil, you have a few things going on here. Make sure your exhaust screen is clear and not clogged,also you are mixing your fuel at 50:1 and when you do you start with an empty can ?And when in use your running at full rpms and not half throttle? Also you could have that build up because the carburetor is supplying to much fuel and its flooding out. 
You may have a bad fuel cap as was stated . the single screw carbs are one turn as you have yours set . If you loosen the fuel cap and let it run for 15 to 20 min and it still does the same thing your issue could be in the carburetor. Is the out side of the carb wet at all ? You could have a leaking welch plug in the carb, or the high speed/main jet valve may not be working correctly. Does this carb only have 2 screws holding the primer bulb and flange on the carb? If so the flange may be distorted and not metering correctly, the newest version of the single screw carb for that unit is c1q-s56 or stihl part #4137 120 0604. Just a few other thoughts for you to work with .
oh and thats a zama carburetor on the fs 80 the only walbro they used on that unit was a three screw high low main idle carb.


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## Captgerryf

*I'll Check out the Suggestions*

Thanks to all for the replies. Tied up this weekend on a variety of issues, but will be cutting grass tonight, so I'll be weed wackin' also.

Just to answer some of your concerns, I always use either Stihl or Echo 2 cycle oil mixed at 50:1. I always use fresh gas and wait till the container is empty before mixing a new batch.

I don't always run at full throttle, just because most of the trimming doesn't require going at it full bore, so that could be the cause of the oily residue as Rotti suggests. Maybe I'll crank it up a bit when doing the trimming.

I'll try the fuel cap vent first and see if that is the issue and if not go into more detail for a problem resolution as Rotti has laid out.

Thanks again.


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## Captgerryf

*Seems to be the fuel cap*

I blew some compressed air through the fuel cap vent hole and ran the trimmer for a good 45 minutes. 

Perhaps glenjudy was right in that 15-20 minutes is enough and time for a break!!!

I did have a few times when I had to actually loosen the cap when it felt like it was bogging down, but it actually ran the entire time.

Besides blowing air through the vent hole should I clean this with some carb cleaner? What is actually clogging this up anyway? Is it just like the varnish that builds up in carb bowls over time? Just curious.

Thanks again.


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## GlynnC

I don't know the reason gas caps gp bad--maybe someone will know. I just replace the cap in this situation rather than trying to clean.


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## rotti1968

That cap is 10.05 for what its worth just replace dont bother trying to clean etc. stihl part #41283500505


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