# FS90R 4 mix woes



## FlyingDutchman (Jul 20, 2020)

Been having trouble with my my FS90R for a couple years now.
It dies at idle, is hard to start and doesn't like to idle down/disengage clutch. I have to crank the idle past clutch engagement so it doesn't die. Sometimes it acts like it is missing and has no power. A tune up kit helped it somewhat a few years ago with dying. The primer bulb is and always has been very, very stiff and always has some air in it. All my other Stihl equipment, the primer pushes easily and lots of times all the air comes out.
A couple weeks ago, I replaced the primer bulb after it broke, and adjusted the valves to spec, adjusted the carb to factory. It fired right up and ran excellent for one or two tanks. I ran it out of fuel yesterday. Put the next tank in and it went back to hell in a handbasket or worse, flooding out instead of running. Double checked the valve cover was tight and spark is extremely strong. Will jump an inch to the motor case.
Takes 20 pulls to get it restarted after shutting off last night, extremely frustrating.
Its possible it got a snoot full of bad fuel, but other equipment seems to run okay on that fuel.
Do these have an inlet screen on the carb?
Do they fall out of adjustment after adjusted?
Time to trade for a Kombi?


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## frank_ (Jul 20, 2020)

sounds like the check valves in the primer are stuck, i just fire another carb on to diagnose them/rule out fueling








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## FlyingDutchman (Jul 21, 2020)

IS that a genuine stihl or a CKO?


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## frank_ (Jul 21, 2020)

FlyingDutchman said:


> IS that a genuine stihl or a CKO?


$10 copy mate, i dont bother stripping carbs if i can help it tbh, easier to fit a new one than pi$$ about


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## Justin Taylor (Jul 21, 2020)

The line from the crankcase to the air filter housing came off


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## FlyingDutchman (Jul 21, 2020)

What do you mean by stripping carbs? Rebuilding them?
Last night it would only run if I pushed the primer bulb, but totally stopped running when i took the fuel filter off. I did have trouble with that same fuel running in other equipment, so I'm going to try some different fuel before swapping the carb. Been using this machine every day so I'd probably just stuff an OEM on it.


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## frank_ (Jul 21, 2020)

FlyingDutchman said:


> What do you mean by stripping carbs? Rebuilding them?
> Last night it would only run if I pushed the primer bulb, but totally stopped running when i took the fuel filter off. I did have trouble with that same fuel running in other equipment, so I'm going to try some different fuel before swapping the carb. Been using this machine every day so I'd probably just stuff an OEM on it.


stripping them down and ultrasonic clean + airline blowing the jets out
it fixes them sometimes, but swapping the carb is easier, and the copy ones are fully adjustable too
oem pistons etc, but stihl carbs are chinese anyway


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## cuinrearview (Jul 21, 2020)

frank_ said:


> swapping the carb is easier, and the copy ones are fully adjustable too


Great plan, until you run into a bad batch of China carbs. Then you're just another "how come my Chinese **** doesn't work and how come they won't help me replace it" thread starter. If you're going to be fooling with small engines, learning how to re-build a carburetor is Day 1 stuff...


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## frank_ (Jul 21, 2020)

cuinrearview said:


> If you're going to be fooling with small engines, learning how to re-build a carburetor is Day 1 stuff...


i fix them for a living


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## angelo c (Jul 21, 2020)

Re check the valve adjustment. Always start with the valve adjustment on a 4 mix


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## FlyingDutchman (Jul 21, 2020)

In Frank's defense, the Stihl dealer I work with said the exact same thing. Replace vs rebuild. Worked like a charm for m FS45. The FS90 has a more expensive carb so it might be worth rebuilding he said, but he said "every time I rebuild one it just seems to bite me in the ass. Replacing works every time." I just don't understand why it would run so well then not at all.

Not looking forward to checking valves again but okay. Thanks Angelo.


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## FlyingDutchman (Jul 22, 2020)

1. Drained fuel. Purged lines.
2. Install new fuel. Purge lines.
3. Remove engine cover. 
4. Remove valve cover. 
5. Set engine to TDC. 
6. Reset valves to .004. They were a bit too tight before Now it feels like it has more compression when pulled over.
7. Install valve cover, engine cover. 
8. Won't start at all. Pulled plug, dried, reinstall. No go.
9. Get mad, go to dealership and buy a carb.
10. Come home. Pull one more time BEFORE putting on carb. Fires up like nothing ever happened runs fine starting on 1-2 pulls.
11. Go insane.


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## lone wolf (Jul 22, 2020)

N\


FlyingDutchman said:


> 1. Drained fuel. Purged lines.
> 2. Install new fuel. Purge lines.
> 3. Remove engine cover.
> 4. Remove valve cover.
> ...


Thats when you buy a new one.


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## angelo c (Jul 22, 2020)

FlyingDutchman said:


> 1. Drained fuel. Purged lines.
> 2. Install new fuel. Purge lines.
> 3. Remove engine cover.
> 4. Remove valve cover.
> ...



Told ya it was the valves.... 
J/k


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## frank_ (Aug 7, 2021)

had one in today with the same hard start and crap idle issues, the valve adjustment was scattering from 5 to 20" thou if you turned it forwards or backwards
it proved to be the hole in the casing that had worn badly oval, letting the cam follower shaft to move up and down
so we drilled the hole out and rammed some alloy tubing in


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## angelo c (Aug 13, 2021)

FlyingDutchman said:


> 1. Drained fuel. Purged lines.
> 2. Install new fuel. Purge lines.
> 3. Remove engine cover.
> 4. Remove valve cover.
> ...


Just adding for anyone following along on adjusting the valves...
There's 2 TDC on these guys..
One where the valves jiggle if you move the flywheel back.and forth and another where the valve don't move if you slightly rock the flywheel. You want the TDC that does NOT move the valves when the flywheel is Slightly rocked.


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## lone wolf (Aug 14, 2021)

angelo c said:


> Just adding for anyone following along on adjusting the valves...
> There's 2 TDC on these guys..
> One where the valves jiggle if you move the flywheel back.and forth and another where the valve don't move if you slightly rock the flywheel. You want the TDC that does NOT move the valves when the flywheel is Slightly rocked.


Are you sure about that?


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## frank_ (Aug 14, 2021)

lone wolf said:


> Are you sure about that?


all 4 strokes are like that


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## lone wolf (Aug 14, 2021)

frank_ said:


> all 4 strokes are like that


Must be some con fusion here! TDC when the rockers have a little clearance V with none at all ,which are you setting the clearance at?
@dmb2613


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## angelo c (Aug 14, 2021)

lone wolf said:


> Are you sure about that?


Very


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## angelo c (Aug 14, 2021)

lone wolf said:


> Must be some con fusion here! TDC when the rockers have a little clearance V with none at all ,which are you setting the clearance at?
> @dmb2613


I use the $4 stihl feeler Guage that comes in the kit with the cork gaskets and a washer


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## angelo c (Aug 14, 2021)

See


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## lone wolf (Aug 14, 2021)

angelo c said:


> See


I just cut on of my feelers with a scissors to fit. The new gasket could be helpful to stop vac leaks .


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## lone wolf (Aug 14, 2021)

angelo c said:


> Very


Huh ! When you are setting the rockers are they a bit loose or totally tight?


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## sean donato (Aug 14, 2021)

lone wolf said:


> Huh ! When you are setting the rockers are they a bit loose or totally tight?


There's only one tdc to set the valves at and it's not the exhaust stroke.


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## angelo c (Aug 14, 2021)

lone wolf said:


> Huh ! When you are setting the rockers are they a bit loose or totally tight?


That little feeler Guage slips in and out snug. There "should" be a bit.of play between the rockers and the spring clip holder thing. It's amazingly simple once you've done a few...I used to hate these motors until I tore a few apart and now I kinda like em...often I get them all locked up and adjust the valves and boom.

Back to work. Or sale...


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## angelo c (Aug 14, 2021)

lone wolf said:


> Huh ! When you are setting the rockers are they a bit loose or totally tight?


I usually shove something soft in the sparkplug hole and get the piston to the top. If the valves are so tight that there is NO play I turn the motor another cycle and see if the valves open and close with the slighted rocking...if they do NOT then you can adjust with the feelers. A couple of "cycles" and the difference is obvious. I'll call you next time I do one...got a couple of pole saws I gotta do soon.
You any good rebuilding the shafts on the ht 101/131 ?
Groundies love bending them up good. $300 a step


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## lone wolf (Aug 14, 2021)

I can do s


angelo c said:


> I usually shove something soft in the sparkplug hole and get the piston to the top. If the valves are so tight that there is NO play I turn the motor another cycle and see if the valves open and close with the slighted rocking...if they do NOT then you can adjust with the feelers. A couple of "cycles" and the difference is obvious. I'll call you next time I do one...got a couple of pole saws I gotta do soon.
> You any good rebuilding the shafts on the ht 101/131 ?
> Groundies love bending them up good. $300 a step


I can do the shafts but i hate them! I know how to set the valves your post was a bit confusing.


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## angelo c (Aug 14, 2021)

lone wolf said:


> I can do s
> 
> I can do the shafts but i hate them! I know how to set the valves your post was a bit confusing.


Yup...it made it simpler for me to think of it this way...everybody always just says "set TDC" and thats not it in this 4mix setup. sorry for the confusion. 
gotta catch up soon.


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## sean donato (Aug 14, 2021)

angelo c said:


> Yup...it made it simpler for me to think of it this way...everybody always just says "set TDC" and thats not it in this 4mix setup. sorry for the confusion.
> gotta catch up soon.


If someone says to set valves at TDC it should be instantly recognized as the compression stroke. Out side of multi cylinder engines where you know which cylinders you can adjust what valves when number 1 is TDC its pretty much the only TDC that matters.


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## angelo c (Aug 14, 2021)

sean donato said:


> If someone says to set valves at TDC it should be instantly recognized as the compression stroke. Out side of multi cylinder engines where you know which cylinders you can adjust what valves when number 1 is TDC its pretty much the only TDC that matters.


Fully agree...however every other *** motor I ever pulled down had compression on EVERY up stroke...these 4mix ers are funky.


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## angelo c (Aug 14, 2021)

angelo c said:


> Fully agree...however every other *** motor I ever pulled down had compression on EVERY up stroke...these 4mix ers are funky.


No idea why it was beeped as I said two stroke or o utdoor p ower equipment.


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## sean donato (Aug 14, 2021)

angelo c said:


> No idea why it was beeped as I said two stroke or o utdoor p ower equipment.


It's because it's the name of another forum


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