Stihl FS 91R factory carb settings?

Jimbo45

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I picked up a used FS 91R the other day, and it runs ok, but the idle was really low, and seemed a little boggy at full throttle. A new plug helped a little, but I wanted to tune up the carb, wondering if the previous owner maybe changed the settings. I cranked the H and L screws in, to see where they currently were at, and the H was at 3 turns out, and the L at one turn out. This seems odd, I've never seen a Stihl carb run an H screw that far out. I guess I could just tune it like I have tuned many other 2 stroke Stihl carbs in the past, but this is my first 4 mix engine, and I'm suspecting the adjustment of the carb may be different than a 2 stroke. Help?
 

MowLife

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The older stihl is 1 turn out on both for a starting point. I believe the new ones are 1 and a half out. Not positive but someone should chime in on it. Also the 4 mix need valve lash adjusted over time.
 

bertsmobile1

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The new carbs all have a narrower taper on the needles so the adjustments are finer.
Thus as previously mentioned most are around 2 turns out where as the old ones were 1 turn out.

But you tune them exactly the same way, screw in till the engine starts to miss lean then screw out till the carb misses rich then set the needle slightly on the rich side of 1/2 way between them.
Set the H first then set the L then set the throttle stop
Go back & readjust the H then the L then the idle again.

Being 3 turns out would not worry me, being 5 or more would.
 

Jimbo45

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The older stihl is 1 turn out on both for a starting point. I believe the new ones are 1 and a half out. Not positive but someone should chime in on it. Also the 4 mix need valve lash adjusted over time.
Yep, the first thing I did was adjust the valves. :smile: They were a tad loose, and it ran a bit snappier after adjusting them.
 

Jimbo45

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The new carbs all have a narrower taper on the needles so the adjustments are finer.
Thus as previously mentioned most are around 2 turns out where as the old ones were 1 turn out.

But you tune them exactly the same way, screw in till the engine starts to miss lean then screw out till the carb misses rich then set the needle slightly on the rich side of 1/2 way between them.
Set the H first then set the L then set the throttle stop
Go back & readjust the H then the L then the idle again.

Being 3 turns out would not worry me, being 5 or more would.
Yeah, thats what I did today after posting the OP. I think I have it pretty spot on, just a touch on the rich side for safety's sake. With that narrow taper on the high speed screw, I did notice there is a much bigger range where it runs well, when adjusting the high speed screw, compared to the Stihl 2 strokes. I ended up with a setting pretty close to 1 1/8 out on the low, and 3 1/8 out on the high. Slight delay when hitting the throttle from idle, but that seems to be the norm with these 4 mix engines. I did notice a slight bit of oil/gas in the bottom of the air cleaner, behind the filter, so I need to check whats going on there, maybe its normal, I dunno.
 

bertsmobile1

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That is normal.
It is called reversion and happens with all carbs more or less.
And yes with the old needles less that 1/2 turn either way would have the engine stall out.
Now days it can be anything up to 2 turns lean to rich.
makes setting them by ear a big problem.

And yes, the acceleration of a 4 cycle always lags that of a 2 cycle so it will not be as snappy as the old ones.
 

Jimbo45

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That is normal.
It is called reversion and happens with all carbs more or less.
And yes with the old needles less that 1/2 turn either way would have the engine stall out.
Now days it can be anything up to 2 turns lean to rich.
makes setting them by ear a big problem.

And yes, the acceleration of a 4 cycle always lags that of a 2 cycle so it will not be as snappy as the old ones.
Thanks for all your help!
 

dfbrown

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I picked up a used FS 91R the other day, and it runs ok, but the idle was really low, and seemed a little boggy at full throttle. A new plug helped a little, but I wanted to tune up the carb, wondering if the previous owner maybe changed the settings. I cranked the H and L screws in, to see where they currently were at, and the H was at 3 turns out, and the L at one turn out. This seems odd, I've never seen a Stihl carb run an H screw that far out. I guess I could just tune it like I have tuned many other 2 stroke Stihl carbs in the past, but this is my first 4 mix engine, and I'm suspecting the adjustment of the carb may be different than a 2 stroke. Help?
I looked at the 2019 stihl reference book fs91 is low speed is 1-3/4 out and high speed is 3-1/2 turns out. Idle is 2,800rpm and ONLY use Bosch usr7ac spark plug gap is .5mm / .020”. Check valves fuel filter and spark arrester. If you adjust valves ever, make absolu sure you buy thr $2.00 kit with the gasket feeler gauge and washe. 4 mix has very low vacuum and if ther is a small leak from re using old gaskets and it leaks it won’t idle for Squat. Make sure the guard is on, get the trimmer normal running temp and make very fine adjustments. When you adjust low speed it will affect idle speed. High speed doesn’t change much like on a real 2 stroke. Good luck. Btw google stihl technical reference guide and look it over. great info.
 

dfbrown

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I looked at the 2019 stihl reference book fs91 is low speed is 1-3/4 out and high speed is 3-1/2 turns out. Idle is 2,800rpm and ONLY use Bosch usr7ac spark plug gap is .5mm / .020”. Check valves fuel filter and spark arrester. If you adjust valves ever, make absolu sure you buy thr $2.00 kit with the gasket feeler gauge and washe. 4 mix has very low vacuum and if ther is a small leak from re using old gaskets and it leaks it won’t idle for Squat. Make sure the guard is on, get the trimmer normal running temp and make very fine adjustments. When you adjust low speed it will affect idle speed. High speed doesn’t change much like on a real 2 stroke. Good luck. Btw google stihl technical reference guide and look it over. great info.
Oh yeah and use a tachometer. If you have one great if not get a wireless stihl EDT8 or EDT9. Not super expensive great to use for any OPE engine. Mowers blowers trimmers etc etc.
 

Jimbo45

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I looked at the 2019 stihl reference book fs91 is low speed is 1-3/4 out and high speed is 3-1/2 turns out. Idle is 2,800rpm and ONLY use Bosch usr7ac spark plug gap is .5mm / .020”. Check valves fuel filter and spark arrester. If you adjust valves ever, make absolu sure you buy thr $2.00 kit with the gasket feeler gauge and washe. 4 mix has very low vacuum and if ther is a small leak from re using old gaskets and it leaks it won’t idle for Squat. Make sure the guard is on, get the trimmer normal running temp and make very fine adjustments. When you adjust low speed it will affect idle speed. High speed doesn’t change much like on a real 2 stroke. Good luck. Btw google stihl technical reference guide and look it over. great info.
Great info, thanks!
 
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