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.

 

[tricky74]

Yes it does appear to have a limiter.
If I run it richer it four strokes.
Running it on 98 RON fuel at 50:1
The revs are a lot higher since I opened up the muffler.
Took me ages to find the air leak at the white ring between motor and carb.
Pushed it pretty hard yesterday and it seems pretty good.

 

[John_Mc]

Yes it does appear to have a limiter.
If I run it richer it four strokes.
Running it on 98 RON fuel at 50:1
The revs are a lot higher since I opened up the muffler.
Took me ages to find the air leak at the white ring between motor and carb.
Pushed it pretty hard yesterday and it seems pretty good.

Four strokes while in the cut? if so, that is too rich. Ideally, it should four stroke a full throttle out of the cut, then "clean up" under load.

Running significantly higher than the recommended octane does nothing other than cost you money, unless you have also modified the saw for higher compression (A muffler mod does not change the required octane.) It can make sense to buy a couple points higher octane as a safety buffer, since the octane rating of fuel can drop over time as various components of the fuel evaporate off while it is stored. For example, if the saw requires a minimum of 89 octane, it can make sense to buy premium (91 or 93 octane, depending in what is available in your area). Buying 100 octane racing fuel is generally a waste of money for a saw with stock compression.

 

[ericm979]

Also if you're pumping from a blender pump (one hose for multiple grades), the first gallon or so is whatever the previous customer pumped. I fill the 5 gallon fuel container for the saws with 91 figuring that it will average out to at least 89.

 

[yomax4]

Also if you're pumping from a blender pump (one hose for multiple grades), the first gallon or so is whatever the previous customer pumped. I fill the 5 gallon fuel container for the saws with 91 figuring that it will average out to at least 89.

Never dawned on me. now I'm kinda pissed. Sometimes I only buy 2 gallons of Non Ethanol and am probably getting E10 from the last guy that pumped. Won't happen again.

 

[tricky74]

Most of the fuel near me is either E10 or 98 the E10 is 94 RON not MON that USA states it in.

Yes I richened it up and it runs like **** and while in a cut I leaned it up until it cleaned up.
It is probably still too rich. It was either four stroking or on limiter pretty much.
Maybe I should lower the depth gauge another 5 thou and retune it again.

 

[John_Mc]

Also if you're pumping from a blender pump (one hose for multiple grades), the first gallon or so is whatever the previous customer pumped. I fill the 5 gallon fuel container for the saws with 91 figuring that it will average out to at least 89.

I buy my non-ethanol (E0) fuel from a station that sells 91 Octane E0 fuel from the same blended pump as their 87 and 89 octane 10% ethanol fuels. When filling cans for my small engines, I use the 91 octane E0 and pump the first couple of gallons into my car.

 

[ericm979]

Maybe I should lower the depth gauge another 5 thou and retune it again.

Depth gauge is different from tuning. I'd leave the gauges at a reasonable level. This is a small saw which will be cutting small stuff. deep gauges gets grabby on small stuff.
(If the teeth have been filed back a ways you might want to check with a progressive depth gauge checker. And make sure the gauge teeth are rounded on the top, not filed flat.)

If it's not cutting well enough with the normal depth gauge setting then the teeth need to be sharper.

 

[yomax4]

I buy my non-ethanol (E0) fuel from a station that sells 91 Octane E0 fuel from the same blended pump as their 87 and 89 octane 10% ethanol fuels. When filling cans for my small engines, I use the 91 octane E0 and pump the first couple of gallons into my car.

Good program. I'll be doing the same !

 

[tricky74]

Depth gauge is different from tuning correct but as it now revs so freely it was on limiter most of the time in a cut.
If I richen it up any it runs like a dog.
This is no longer what we would call a stock saw.

 

[yomax4]

I'll have to look up the rev limiter feature. I wonder what the rpm's are when the limiter kicks in. No limiters on any of my saws as far as I have noticed. All have muff mods and re-tuned. When I did the muffler mods all that was needed was about a 1/8 turn counterclockwise on my high screw and then adjust idle. Yours seems haunted.