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Stihl TS 420 Won't hold idle unless primer bulb is pushed


johnjuniet's Avatar
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09-07-12, 04:34 PM   #1  
Stihl TS 420 Won't hold idle unless primer bulb is pushed

I was given a Stihl TS420 Cutting Saw to fix. It had pure gasoline run through it, and it would no longer start. I have so far replaced the piston, ring and cylinder assembly, spark plug, hoses, gaskets and carburetor. I have attempted to adjust the idle with the old carburetor, but not with the new one (assuming that it's set to factory standards). The persistent issue is that when it starts, it won't hold a low idle, nor will it stay running unless the primer bulb is pushed continuously. When pushed, the fuel will run the motor until the amount of fuel in the carburetor wanes, then if left alone, it will die out. This has occurred at every turn in the process, and still persists.

I'm not sure where to go from here. What could I be missing? What else can I check/change/repair/replace?

Is there a diagram anywhere that shows how the hoses are supposed to be routed (in case I have them backwards), because I think I have the hoses right, but I am only human. I have checked youtube for backyardmechanic and for donnyboy73, but neither address this issue with this particular saw. Any help or direction you can provide would be greatly appreciated. Thanks.

 
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09-07-12, 05:04 PM   #2  
It just sounds like the idle circuit needs cleaning. If you have a new carb for it and plan on using it, I would install the the new one and see where you're at.

Or you could dismantle the old one and clean it - it may work fine then.

 
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09-07-12, 08:39 PM   #3  
Sounds to me like the fuel pump in the carb isn't working, which could be due to leaking pressure in the crankcase, and the inability to idle goes right along with that diagnosis. When one gets run with straight gas, the crankshaft seals can get cooked. The bearings on the crank might even be cooked and allowing the crank to walk a bit. See if you can make it run better by spraying carb cleaner or starting fluid at each of the crank seals while it is running. If so, they are leaking.


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God bless!

 
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09-21-12, 03:16 PM   #4  
Thank you. I did replace the carb anyway. Where do i / how do i finx the idle circuit?

 
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09-21-12, 03:20 PM   #5  
Thank you. I replaced the carb since this post, and the problem continues. I will check the seals and see where that gets me. What are your thoughts on tge previous post regarding the idle circuit?

 
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09-22-12, 12:11 AM   #6  
The idle circuit is a passage in the carburetor.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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10-19-13, 07:18 PM   #7  
Stihl TS 420

I have the same unit, I have rebuilt the carb with the recommended carb kit, new plug, new fuel lines. What I found was a very tiny pin hole in the primer bulb allowing the system to suck air, I have yet to replace this to see if this solves my issue. I will keep you posted as to my findings.

 
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10-19-13, 07:34 PM   #8  
Changed the fuel filter In the fuel tank?
Tried running it without the fuel cap in place?
Done a compression test?

 
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