Troubleshooting: Stihl saw won't idle (dies)

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  #1  
Old 12-28-09, 08:29 AM
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Troubleshooting: Stihl saw won't idle (dies)

Hi, all.

I've been troubleshooting an idle problem with a Stihl saw (TS-400 cutoff saw) and I'm nearly out of ideas. Hoping someone here can help. I feel invested now and don't want to simply hand it over to the service department.

By the way, I bought this saw used, so I don't know anything about its history.

The saw runs fine at higher speeds, but if I try to let it throttle down it dies immediately below a certain rpm. It also hesitates when I throttle up (does not accelerate responsively -- may actually drop a little for a half-second).

Making sure the saw is well warmed up does not help.

It was interesting that earlier, I could adjust the saw so that it would idle for a little while (less than a minute). Trying to solve that, I disassembled the carburetor, gave it a dip treatment, and reassembled with a full carb rebuild kit. I also adjusted (bent down) an inlet control lever ever so slightly to sit flush with the carb chamber floor, just as it is supposed to. But afterwards the idle performance was actually worse, and no carb adjustment corrected that.

I disassembled the carb and bent the inlet control lever the other way a bit, but that had little if any effect.

The saw passes all 3 vacuum and pressure leakage tests for the carb and crankcase.

The spark plug was replaced not long before I got serious about solving this.

Also replaced:
- a fuel hose
- in-tank fuel filter
- auxiliary air filter
- spark arrest screen
- exhaust gasket
- decompression valve (lets the Stihl start easier with less kick-back)
 
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Old 12-28-09, 11:00 AM
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I don't know if this is recommended or not, but after a dip in the treatment did you blow the carb out with an air hose?

I would think sometimes just a dip in the treatment will not get some fine particles out of those tiny holes.

Just a thought...Good Luck!
 
  #3  
Old 12-28-09, 11:42 AM
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Originally Posted by Honda 327 View Post
I don't know if this is recommended or not, but after a dip in the treatment did you blow the carb out with an air hose?

I would think sometimes just a dip in the treatment will not get some fine particles out of those tiny holes.
I'm pretty sure I did, but maybe I'll have to revisit the condition of the idle fuel port.
 
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Old 12-28-09, 03:41 PM
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What is the compression reading for this unit ?

Here's a link for tuning the carb:
Saw Carb Tuning
 
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Old 12-28-09, 06:38 PM
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I was wondering what compression it has as well. Compression is a common problem area on cutoff saws from all the concrete dust ingested by the engine.
 
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Old 12-31-09, 11:59 AM
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Compression is 120 psi. Does that tell you anything?
 
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Old 12-31-09, 01:02 PM
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The compression at 120 is fine - the problem is either in the air flow throughput (inlet filter or exhaust) or the amount of fuel pulled into the air stream at idle. Considering what you've done and checked, it's probably the latter.

Start with the idle mixture screw set out at 1 1/2 turns from a light seat. Then work it all the way back in. If the circuit is normal it should speed up, then die at anything that resembles an idle. If no change, start at the 1 1/2 turns out and go the other way. If does nothing, the idle circuit is messed up.

It could either be plugged or flooding to max = ineffective. If your kit went in OK and you have it cleaned out well, you may have to invest in a new carburetor.
 
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Old 02-24-10, 11:59 AM
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Talking

Originally Posted by marbobj View Post
The compression at 120 is fine - the problem is either in the air flow throughput (inlet filter or exhaust) or the amount of fuel pulled into the air stream at idle. Considering what you've done and checked, it's probably the latter.

Start with the idle mixture screw set out at 1 1/2 turns from a light seat. Then work it all the way back in. If the circuit is normal it should speed up, then die at anything that resembles an idle. If no change, start at the 1 1/2 turns out and go the other way. If does nothing, the idle circuit is messed up.

It could either be plugged or flooding to max = ineffective. If your kit went in OK and you have it cleaned out well, you may have to invest in a new carburetor.
Good call, Marbobj. On account of obvious wear on the piston skirt (worn semi-circular area on cylinder below intake port), I installed a new cylinder-and-piston assembly, but that did not solve the problem.

Though the carburetor low-speed ports did not seem to be plugged (squirted carb cleaner through with no obstructions) and I had already done a carb soak and installed a carb rebuild kit, I had other advice from a mechanic that works on Stihl saws who said that despite all that, this was probably a carburetor problem. He sees it often with the Stihl cutoff saws. He advised that I simply replace the carb at this point.

I did, and now it idles beautifully!
 
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