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Discussion Starter #1
Guy's,

I am working on A Stihl FS80 brushcutter that has a weird problem I cant pinpoint. tell me what you think i should check...

the unit starts easily, stumbles just a wee bit on revving to full throttle but gets to WOT and runs good.. if you run it wide open throttle for several minutes it starts to miss/stumble and change RPM but doesnt shut off.

I put an inline spark checker between the plug and wire and viewed the spark while running. it looks like the spark is not changing while it is throwing it's fit. could I have a fuel / carb problem??? I dont think it's an air leak because it does idle and is pretty easy to start. maybe clean carb and adjust metering lever???

what do you think??

Thanks,

Mark
 

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Could be the fuel filter is getting dirty and not able to supply the fuel fast enough or the fuel cap could be starting to fail and not venting properly, the metering arm is the last thing I would adjust as it has the proper height setting when it was running properly and can be touchy. So the usual, spray carb cleaner along all mating surfaces while running to see if engine tempo changes, remove muffler and check for carbon buildup in the arrestor and exhaust port, open the H by 1/8-1/4 turn. Have a good one. Geo
 

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just so you know there is and are in most of stihls units a rev limiter built into the ignition modules. At wot the unit should start hitting it and will continue to until you let off of the throttle or put the unit under load . Before you go chasing something you may not have an issue at all.
 

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Discussion Starter #4
Rotti,

you know, I've read about Stihls and the rev limiter , but this thing runs pretty well for the first 3 or 4 minutes , then starts it's missing/stumbling fit.

how does a unit act that has a rev limiter? does it just wind out RPM's and then level off and run smoothly??
 

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That is a very good question, I have a BG75 which doesn't have the limiter, a FS85 which does. They have the same engines as the FS80, however the blower doesn't need the limiter because of the blower fan so it doesn't rev as high. When the weather warms up(next summer) I will wind up a FS85 without the trimmer head and let you know,(LOL) for now you will have to assume it is another type of problem. Have a good one. Geo
 

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An engine with an electronic rev limiter, will run rough or appear to have an ignition miss when the limiter is activated. Once a load is applied or the throttle lowered below the threshold, the engine will smooth out and run properly.
 

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Discussion Starter #7
Thanks 30Year,

that is exactly the way it acts. I guess i can be glad I asked you guys before I went and tore it apart looking for a problem that dont exist.

so answer me this,

why do we need a rev limiter when you can only squeeze the trigger so far and make it run so fast?
 

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I do not mean to step on toes here. I do not know if the newest FS 80’s have electronic speed limiting built into the ignition coil but I know units sold before 2005 did not. I have the service manual and it shows maximum speed as a function of the carburetor.

Is the deflector shield installed, to control line length?

Either the carburetor needs some tweeking or there is possibly some blockage.
 

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so answer me this,

why do we need a rev limiter when you can only squeeze the trigger so far and make it run so fast?
That's because when the trigger is squeezed as far as it will go, the carburetor is usually wide open. The throttle linkage on most units is direct to the carburetor, without any mechanical speed governor. Depending on the design of the engine, with no load it may be possible for the engine to run faster then it should, causing it to sling the lubricant from the crankshaft and connecting rod.
 

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I do not mean to step on toes here. I do not know if the newest FS 80’s have electronic speed limiting built into the ignition coil but I know units sold before 2005 did not. I have the service manual and it shows maximum speed as a function of the carburetor.

Is the deflector shield installed, to control line length?

Either the carburetor needs some tweeking or there is possibly some blockage.
Starting with tech bulletin 41.97 (1997) they started using electronic speed control on the fs 80s.if the coil is marked with with part number ending 1306 or 1307 it has the esl built into the module.this started on serial #x37 759 999. part #4137 400 1350 is the most recent module but it comes in a kit form and is coil # ending 1307 and mounting hardware .
 

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An engine with an electronic rev limiter, will run rough or appear to have an ignition miss when the limiter is activated. Once a load is applied or the throttle lowered below the threshold, the engine will smooth out and run properly.
Could not have said it better.....
 

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Starting with tech bulletin 41.97 (1997) they started using electronic speed control on the fs 80s.if the coil is marked with with part number ending 1306 or 1307 it has the esl built into the module.this started on serial #x37 759 999. part #4137 400 1350 is the most recent module but it comes in a kit form and is coil # ending 1307 and mounting hardware .
I stand corrected and thanks for catching my error. I did not remember FS 80’s having the electronic speed limitation and I forgot to look at the TI’s in Stihl’s Service Communications System (SCS).
 

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the unit has a brushcutter blade on it, think that may have anything to do with it?
Yes, it will be much more noticeable with a blade installed. Trimmer heads with string have more load on the engine from spinning the trimmer line through the air. Blades have very little aerodynamic resistance and this allows the engine to accelerate faster and have the limiter reaction seem more sensitive.
 

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Discussion Starter #16
Today it is supposed to be around 40 degree's out, it has been in the 20's. I will take this brushcutter out to the back 40 and see how it acts doing some actual cutting.

if everything seems well were going to call it good. I did replace the plug and put fresh gas in it before I posted here.

Thanks to all who responded, I'll let you know what happens...

Mark
 
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