MS171 tune changes daily

[tricky74]

HI I have an MS171 that I opened up the muffler on. It starts and idles fine. Low end seems fine.
One day it screams and cuts quite well. Next day the top end is off and you can hear it is not right.
Only takes a slight adjustment to get it singing again.
But I just leave it as is and use it that way.
Any ideas on what it could be? I have had the carb off a couple of times to checkout is clean.

 

[ericm979]

Did the ambient temperature change a lot between the days? If so, that's not so unusual. If not it might mean that the carb adjustments are way off. You can get it so it runs ok but it requires frequent fiddling. Go back to the factory recommended default settings (usually around 1 turn out for the mixture screws for Stihls) and follow the recommended adjustment procedure from the manuals. The owners manual has a basic procedure, the shop manual has a better one. Or you can follow this one: set the mixture screws to the default. Adjust the idle screw to the highest idle speed then richen it a little. Adjust the idle stop to get the right speed. The best way to adjust the high speed screw is to cut with the saw. When you lift a little out of the cut the saw should four stroke some but clean up when it's cutting. If it doesn't four stroke when you lift it's too lean, if it does it in the cut it's too rich. This gets you a safely rich mixture.

 

[fruitcakesa]

Are there O-rings on the screws?

 

[yomax4]

The muff mod will require a different tune. just don't let it scream while too lean. Maybe O-rings as stated above. You want it to 4 cycle when you rev it until you get into the cut. You can hear it going in and out of 4 cycle. Great Youtube vids on this. Good Luck.

 

[tricky74]

I will check the orings. Changed the plug today as well. It is pretty hard to tune as it runs near rev limiter.
Full chisel chain and muffler mod. Currently no spark arrestor as it is used away from the bush and was clogged.
Seems to run well when I have one of my pro saws as a backup and ready to go.

 

[dodge man]

Take a peak at a thread I started. I messed with the mixture screws a never could get it right. When I cleaned my muffler I looked in the exhaust port and have a scored piston. The thread is titled MS250 mixture screws.

 

[tricky74]

Update.

Changed spark plug and no different.
I am pretty sure it is just the sound of being on limiter a lot.
Have lowered the depth gauge / rakers and it it a lot better but still on limiter in softer or smaller wood.
Actually cuts quite well. I think it likes the open muffler.

 

[yomax4]

Update.

Changed spark plug and no different.
I am pretty sure it is just the sound of being on limiter a lot.
Have lowered the depth gauge / rakers and it it a lot better but still on limiter in softer or smaller wood.
Actually cuts quite well. I think it likes the open muffler.

The MS 171 has a rev limiter?

 

[yomax4]

Update.

Changed spark plug and no different.
I am pretty sure it is just the sound of being on limiter a lot.
Have lowered the depth gauge / rakers and it it a lot better but still on limiter in softer or smaller wood.
Actually cuts quite well. I think it likes the open muffler.

The MS 171 has a rev limiter?

 

[John_Mc]

Are you running the same fuel each time? Ethanol gas (E10) can cause you to run slightly leaner (since it's an oxygenated fuel) than non-ethanol gas (E0). Some folks just tune their saw to run well on E10, and then don't adjust when running E0 - it will run a little rich on the non-ethanol, but the consequences of running slightly rich are not as bad as running too lean. The difference between the two is not huge. So to cause you a significant problem, I'd expect you'd need to be tuned fairly close to the edge for a switch from E0 to E10 to cause you such a noticeable problem.

One other thoughts: If you are constantly bumping up against the rev limiter, there is a good chance you are running too lean. You could be simply set too lean on your mixture, or you may have an air leak (sometimes an air leak comes and goes - or the severity changes). If it is an air leak, that's something to get addressed ASAP as an air leak can cause you to run too lean and seize up your saw.