Help needed: Stihl MS 180C Chainsaw idle issue

[ahdofu]

Hello. I have a problem with my Stihl chainsaw. When idling, it eventually dies. Past idle it works fine and the throttle action is fine as well. I tried to increase the idle speed but that screw has no effect! Looking at the carb, the idle screw does not work like a traditional screw where it presses against the throttle plate. The idle screw actually moves in and out of the carb itself. I removed the carb, and inspected the inside inasmuch as I can. Rubber valves seemed OK. Put is back together but the issue persists. To me it seems like there is a leak somewhere. Has anyone come across an issue like this and found a solution?

 

[Vintage Engine Repairs]

 

[ahdofu]

Hi JAXJEREMY,
How do I perform the perform the pressure and/or vacuum test?

 

[ahdofu]

Just to close this tread I finally managed to get the issue resolved. Once I ruled out the fuel line having a leak, I replaced the diaphragms in the carb and that did it. Now the engine reacts to the idle screw position and all is well. I cannot readily tell what was wrong with the old diaphragms. They are both flexible and yet something must have been borderline with one or both.

 

[HarleyT]

You likely dislodged some crud in there, or got fresh fuel in the system.

 

[ahdofu]

No I had previously cleaned the carb w/o replacing the diaphragms and was using fresh fuel. The problem still persisted.

 

[Wood Doctor]

Could be that the carb's low-speed setting was set too rich. Not enough air at low speed means it will run OK for awhile and then get drunk on fuel and drown.

 

[Oletrapper]

Putting my money on the metering diaphragm. If the least bit stiff it won't function as intended. jmho OT

 

[HarleyT]

Here should be the carb in question. Note the "adjustment" screw.

 

[Oletrapper]

Here should be the carb in question. Note the "adjustment" screw.View attachment 978484View attachment 978485

Appears to be EPA at its finest. lol jmho OT

 

[Wood Doctor]

Agree with Oletrapper. I ran into this "no-setscrew" carb situation three years ago with a 180 and replaced it. After that the engine ran fine and the owner uses it all the time. Nice Pics, HartleyT.

 

[ahdofu]

I don't think folks have completely read this thread in its entirety. Summarizing:

1. MS180C dies in idle after a while. However high speed & throttle action are fine. This saw has always been fueled with Stihl's remixed fuel.
2. Tried to increase idle speed using the idle screw (it is a reverse thread screw). It had no effect. Put the screw back where it was.
3. Checked fuel filter. It was fine.
4. Tried fresh fuel. No effect when ran the engine.
5. Cleaned air filter. Was a bit dirty. No effect
6. Removed carb. Both diaphragms were flexible and seemed OK. Used clean streak to spray clean the carb. Put it back together and ran the engine. No change in behavior.
7. Checked the chain clutch and its brake (should have done this in the beginning). However both were OK. Ran the engine. No change.
8. Removed the fuel tank and the fuel line. Tested the fuel line. It was OK. Put it back together. Ran the engine. No change.
9. Removed the carb again. This time replaced both diaphragms. Ran the engine and all was back to normal. Also confirmed that the engine reacted to the idle screw position.
10. My guess is that one or both of the old diaphragms were borderline.