# Stihl KM90r won't idle when warm



## walterg

My KM90r has been sitting for roughly four months or so with gas in it.
I emptied the old fuel out and put fresh mix in.
Started harder then normal then settled down to an idle. High rpm sounded fine. Idle was okay, maybe a hair low.

After five minutes or so it won't idle anymore. Hot restart was normal.

When I last used the KM90r it had fresh 93 octane non ethanol gas with Stihl Ultra mix. 

It has never had ethanol fuel in it. I only use 93 octane non ethanol gas with Stihl Ultra mix.

So I guess I need a carb kit.? Any special tools needed, or any advice.?
It was bought new in early 2015, and has less then 20 hours on it.

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## Conquistador3

20 hours is a little early for a carb rebuild, even if you let fuel in it, but it's not impossible. 

You forgot to mention if it starts right away when hot or needs to be left alone for a few minutes and if it dies right away after letting the gas go or it just sputters before shutting off.


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## walterg

Starts right away when hot. It dies instantly when you let off throttle.
Cold start yesterday took 12 + pulls.

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## Conquistador3

It sounds like it's running way too lean... have you adjusted the carburetor since buying it? Perhaps leaned the "low speed" mixture?


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## walterg

Just tried starting it and it started within 4 pulls. Let it idle and it cuts off at four minutes.
To restart you need to hold the throttle open. It cuts off as soon as you let go of it.

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## Conquistador3

OK, all clear now. If you haven't touched the carburetor settings, you need to service the carb, meaning it needs a new set of membranes and a good soaking in carburetor cleaner (pure acetone will do just fine). 

Please check before buying, but I believe the KM90 uses a Zama C1Q-S1** carburetor. Zama carburetors don't respond as well as Walbro to rebuilds, so you may want to try a cheap Chinese carburetor instead, as they are truly ridiculously cheap these days.


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## walterg

It has the Zama. I tried cleaning it with no luck. Still runs the same.

I ordered one of the China carbs, hopefully it will be here Friday. I also ordered a carb adjust tool.

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## ANewSawyer

Have a look inside the chinese carb before you put it on. I found the screen not in place on mine.


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## walterg

Thanks. I got my carb on Saturday. It's a good one, no issues.
My carb adjusting tool came Monday. 

It started on the first pull with the new carb. Low end seems fine, but the high side needs adjusting.
Hopefully be able to do that on Thursday.

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## Conquistador3

Results!!!


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## Highdesignfool

walterg said:


> My KM90r has been sitting for roughly four months or so with gas in it.
> I emptied the old fuel out and put fresh mix in.
> Started harder then normal then settled down to an idle. High rpm sounded fine. Idle was okay, maybe a hair low.
> 
> After five minutes or so it won't idle anymore. Hot restart was normal.
> 
> When I last used the KM90r it had fresh 93 octane non ethanol gas with Stihl Ultra mix.
> 
> It has never had ethanol fuel in it. I only use 93 octane non ethanol gas with Stihl Ultra mix.
> 
> So I guess I need a carb kit.? Any special tools needed, or any advice.?
> It was bought new in early 2015, and has less then 20 hours on it.
> 
> Sent from my LGLS770 using Tapatalk



Wouldn't that still be under warranty?


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## walterg

Actually your right. I looked up the warranty and its 2 years. 

If I took it to a dealer...
They probably would of denied the warranty and claimed it was a fuel issue.
Anyway its fixed and running great.

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## Highdesignfool

walterg said:


> Actually your right. I looked up the warranty and its 2 years.
> 
> If I took it to a dealer...
> They probably would of denied the warranty and claimed it was a fuel issue.
> Anyway its fixed and running great.
> 
> Sent from my LGLS770 using Tapatalk



I'd still try to get it fixed under warranty. At least you would have a proper Stihl carb


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## walterg

So much for running right. I'm on carb three, and I've just about given up.
The problem is I've got too much invested in the Kombi system to sell it off.
Km90r
Straight shaft trimmer
Straight shaft brush cutter
Pole saw and extension
Adjustable hedge trimmer

Not sure what too do guys, their is no money in the budget for another power head, and would it be better then the first one.? 

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## Conquistador3

May I ask you if symptoms are still the same or there's something new?


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## walterg

Conquistador3 said:


> May I ask you if symptoms are still the same or there's something new?


It's worse. Still has an idle problem, but now it won't open up.

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## Conquistador3

I don't understand what you mean by "open up"... you mean the choke doesn't work? 

I don't know how many hours you have on that powerhead, but 4-Mix engines are well known to suffer from valvetrain related fuel issues. Valves need to be regularly adjusted and use of crappy premix oils (which include Stihl's own products: they are overpriced junk) will lead to carbon buildup which interferes with valve operation. Early BR600 engines were notorious for this behavior, among other things. 
If you don't know how to check the valvetrain or don't trust your skills, and don't want to deal with a Stihl dealership, put an ad in the Chainsaw section. plenty of skilled people here that can give a look and you may find somebody local.

Finally, give the impulse line a good look. Those things may cause all sorts of fueling issues when they start leaking.


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## walterg

It still has less then 20 hours on it. It has only run Stihl Ultra mixed with 93 octane non ethanol fuel bought from a high volume station.
I will check again but impulse line seemed fine.
By not opening up I mean it doesn't want to hit high rpm's, like the carb is out of adjustment.

Carb replacement 1 which ran so good started doing it. Carb 2 was possibly defective, but carb 3 is acting exactly like carb 1.

I will get it out the week after Christmas and try it again.

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## Conquistador3

walterg said:


> It still has less then 20 hours on it. It has only run Stihl Ultra mixed with 93 octane non ethanol fuel bought from a high volume station.
> I will check again but impulse line seemed fine.
> By not opening up I mean it doesn't want to hit high rpm's, like the carb is out of adjustment.
> 
> Carb replacement 1 which ran so good started doing it. Carb 2 was possibly defective, but carb 3 is acting exactly like carb 1.
> 
> I will get it out the week after Christmas and try it again.
> 
> Sent from my LGLS775 using Tapatalk



Stihl has had occasional issues with fuel and impulse lines, not to mention those crappy cheap Zama carburetors. This still sounds like a fuel delivery issue, sorry about it.
At this point it sounds to me like this thing should have been tackled by the dealership which sold it from day one: Stihl 4Mix are the most temperamental engines I've seen from a major equipment manufacturer. Some will last forever with nothing more than regular valve adjustments while others will have valvetrain issues, suffer from carbon buildup at relatively low hours (obviously while using Stihl's marvel-oil), spit fuel into the air filter etc. 
I have three Stihl chainsaws at home, all bought brand new, so I am no hater, but I have long wondered why the good folk at Waiblingen felt the need to reinvent the wheel.


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