Stihl MS-181 spark plug

[coppicer]

My little Stihl MS-181 starts from cold without problems, but it often has difficulty picking up off idle when it gets fully warmed up. If I just squeeze the throttle, it will often die unless I back off quickly, so I need to open it carefully and little by little until it picks up speed. It's like finding the exact edge at which it bite and go. Once it's wide open it's fine.

 

The air filter looks OK. I have never replaced the spark plug, but I've had it for six years now so I thought I would do that. A picture of the old one is attached; looks a bit mucky to me. Maybe I messed up the fuel/air mix at some point? Is there a thread I can refer to for zeroing the carb and starting from default values for the two screws?

 

Thanks!

 

 

Edited April 17, 2020 by coppicer
Spelling

[openspaceman]

41 minutes ago, coppicer said:

My little Stihl MS-181 starts from cold without problems, but it often has difficulty picking up off idle when it gets fully warmed up. If I just squeeze the throttle, it will often die unless I back off quickly, so I need to open it carefully and little by little until it picks up speed. It's like finding the exact edge at which it bite and go. Once it's wide open it's fine.

 

The air filter looks OK. I have never replaced the spark plug, but I've had it for six years now so I thought I would do that. A picture of the old one is attached; looks a bit mucky to me. Maybe I messed up the fuel/air mix at some point? Is there a thread I can refer to for zeroing the carb and starting from default values for the two screws?

 

Thanks!

 

 

Looks ideal mixture strength to me

[spudulike]

From the symptoms it sounds like lack of fuel. You could try turning the L screw a 1/4 turn anticlockwise - that is if your carb has the L&H screws and isn't the single screw one.

If this doesn't work, it could be a blocked gauze filter in the carb or dirty fuel filter. You don't say what age the saw is but if it is an old one, the fuel line could be split or the carb diaphragms may be past their best.

The plug colour you have got just tells you that flat out, the saw is running as it should and not that everything else is OK which probably rules out a split fuel line.

[Heavy Oil Saw]

If the 181 is like the 171, the carb has turn limiters on the adjusting screws. I cut mine off and adjusted from there, I copied the procedure from a youtoob video.

Sorry, deleted as already said above. I’d put my money on the carb diaphragm.

Edited April 17, 2020 by Heavy Oil Saw
Late posting.

[coppicer]

44 minutes ago, spudulike said:

You don't say what age the saw is but if it is an old one, the fuel line could be split or the carb diaphragms may be past their best.

Thanks, it's about six years old. It doesn't get heavy use, but it does a bit every year. Using it a bit more at the moment, which is why I noticed the pickup issue. Will take a look at the points you raise, starting with the screw.

[bmp01]

On 17/04/2020 at 19:57, coppicer said:

Thanks, it's about six years old. It doesn't get heavy use, but it does a bit every year. Using it a bit more at the moment, which is why I noticed the pickup issue. Will take a look at the points you raise, starting with the screw.

And there's the clue. How fresh is the fuel ? Do you drain fuel and run saw dry before storing ? Evaporated fuel causes havoc with carburettors but as its still runnIng you might get lucky with fresh super unleaded and semi synthetic 2 stoke oil.

 

Edited April 18, 2020 by bmp01

[coppicer]

8 minutes ago, bmp01 said:

Do you drain fuel and run saw dry before storing ? Evaporated fuel causes havoc with carburettors but as its still runnIng you might get lucky with fresh super unleaded and semi synthetic 2 stoke oil.

A good point. This belatedly occurred to me a couple of years back, so I switched to using either Aspen or Stihl MotoMix and I don't use conventional petrol in my saw at all any more. But might there be residues in the carb built up from the time before I switched to low-ethanol fuels...?

[Heavy Oil Saw]

When was the saw last serviced? As in more than a air filter and plug. 6 years on varying fuel will of tired the rubber out, on top of the work they do. Get it disassembled, service the carb, and inspect everything else, and see how she runs afterwards.
I didn’t want to risk problems when I assembled a saw, so renewed all the rubber items and serviced the carb, no running problems. Even renewed the throttle rod, can’t believe how they wear out against plastic.

[Dan Maynard]

Switch from years of petrol to aspen will leave the rubber hard, that's a known issue due to the effect of the petrol.

Afraid it possibly needs carb service.