MS170 Chain Spinning at Idle

[mitchstockdale]

Hi all,

I have a 10year old Stihl Ms170 that I have been using lately and ran into a problem.  

A couple months back I was cutting up some spruce that had some stringy center rot and the chain quit spinning while in the cut...clutch area was noticeably hot so i shut it down.  This seemed odd since the spruce was nothing out of the ordinary...I just chocked it up to stringy fibers and different densities of wood.  I had also just started using a different brand bar oil and removed the clutch cover and blew out behind the clutch with compressed air...I cant see this making a difference.

Since this incident the chain has started to turn while the saw is idling (faster than typical creeping) and seems to pick up speed once the saw heats up. I have been researching the problem for about a month and have completed the following:

- Cleaned out behind the clutch.
- Removed the clutch and hit all the parts with a wire brush as there was some mild rust in a few places as it appeared like one clutch shoe was harder to move.  Put the clutch back together with a little anti-seize (obviously not on the face of the shoes)
- replaced the clutch springs.
- Although the idle was normal I turned down the idle just to be sure this just stalled the saw and didn't stop the chain.
- Checked the carb for loose bolts and air leaks around the filter...didn't find any noticeable problems.

Recently had the gas tank vent start leaking..likely just worn out.

I am at a loss and don't know what else to check.  Hoping to draw on some of the forums knowledge.  Saw cuts great despite the minor chain spinning safety issue.

[sawguy21]

Did you grease the sprocket bearing? It may be seized, a little dab of white grease will do it.

[mitchstockdale]

@sawguy21 , yes sir I have greased it recently.  I guess I could try replacing it, although, it looks to be in good shape. 

[WhitePineJunky]

Chains will spin at idle when the clutch weighted springs are worn 

[thecfarm]

Many years ago, I had the same problem.
I don't work on my saws, but the saw Dr fixed it, and I paid the bill.
I forgot what it was.

[lurk]

I am at a loss and don't know what else to check. 

While you had the clutch off, did you check the oil pump worm drive gear? If this is stiff on the crank it could also turn the clutch drum until they fail completely.

Part number 1123 640 7102



 

[mitchstockdale]

@lurk  I did not check the worm drive, it makes sense, i will do so today.  Thanks .

[mitchstockdale]

@lurk  I checked the worm drive.  I tried spinning it and it seemed like it should rotate easier... I cleaned the inside of the copper bushing with a wire brush and added some grease.  it turned a bit easier after that but not by much...i reassembled and tried it...chain is still spinning..but i think will try a new worm drive.  

[Keepfixin]

Sounds like possibly an air leak from a crank seal. If you could post a picture of your spark plug it would tell a lean or rich mixture. Also did you ever blow out from around your carb with your nozzle? I used to blow off my carbs from underwater pressure testing with compressed air. But don't do that anymore because I ruined two carburetors check valves blowing them dry.  So they didn't work right after that.. :-/

[mitchstockdale]

@Keepfixin ,  I think you are right about the air leak.  I was in the parts store and the guy working the counter said they see 170's with leaks quite often.  So its looking like a tear down.  

Thanks everybody who commented, very helpful.  I will report back what i find when i dig into it.

Thanks again

[Keepfixin]

Ya it seems like that's a common issue with all homeowner saws anymore. Worked on 3 ms290's in the past year and a half and the seals were out and two of them bearings too. But the two of the three saws had for sure polyamide caged bearings which would mean the cage starts wearing out, then the bearings get loose, then your poor old seal gets wiped out, and then your saw spits out the melted cage out your exhaust!!!!!
So maybe give a wiggle on the end of the crankshaft to see if you have any sort of play. If you do a complete tear down buy some metal caged skf or koyo bearings :-)

[lurk]

- Although the idle was normal I turned down the idle just to be sure this just stalled the saw and didn't stop the chain.

Was the reason I didn't go the air leak route.