Stihl MS 271 Diagnosis

[Sam R]

Stated complaint: Customer brought in non-running saw, stated it ran fine until it died mid cut. Would not restart. Customer claims it is a carburetor issue.

Initial visual exam and test run: Unit looks to be clean, well cared for. Engine pulls over fine; piston is not frozen, compression feels a bit low. Will not start following correct starting procedure, also won't start w/ WOT.

Diagnosis: Following Stihl engine check sheet the full diagnosis reveals a few interesting facts about the unit:

• The saw has approximately 110 lb of compression
• The intake side of the p&c are scored across the skirt
• Rings are free, exhaust side looks okay
• Unit fails vacuum (negative pressure) test, but passes positive vacuum test except:
• Bad bearings in unit mean lateral play in crankshaft is possible & will release all held pressure
• Which means the seals (primarily on clutch side) are bad
• Intact limiter caps on Carburetor rules out tampering; carburetor throttle shaft okay, carb passes positive pressure test; unit failure conditions not in line w/ bad carburetor

Analysis: The chain on the unit tightened to the point of being nearly impossible to turn by hand w/ brake released. Clutch sprocket somewhat worn. Likely cause of failure seems to be chain tightness pulling laterally on ball bearings and causing failure and introducing secondary air leak leaning out mixture until the scoring was bad enough to halt the unit. The problem was probably not as apparent as general seal failure (surging, poor idle & other typical issues). Unit died mid cut when it could no longer generate enough +/- pressure to operate fuel pump & transfer ports.

Conclusion: The saw is toast; the cost to repair the unit borders on $400 making repair economically unwise.

 

[Chainsaw Jim]

The saw was ran with either a clogged filter or with it removed. It ingested fine particles which will score the intake side of pistons and also cause bearing problems. The bearings will start running hot enough to weaken seals.

 

[Sam R]

It came in with a pretty clean filter & no evidence of dirt ingestion on choke butterfly or venturi. Although particulate ingestion could explain the lowish compression since that will erode the piston walls. It doesn't have the trademark dull, sandblasted look though. I still think it was the chain being too tight.
It's the same principle of bearing failure in power steering pumps & compressors in cars when the belts are put on too tight by a mechanic (or homeowner) who decides his thumb is a good deflection measure.

 

[mountainlake]

With the intake side scored it took in some dirt, running lean scores the exhaust side, straight gas will score all the way around the piston. Steve

 

[Sam R]

There's no evidence of dirt ingestion; I didn't see any dirt down in the crankcase, the connecting rod main bearing isn't gritty, the intake, clean side of filter - all perfectly clean. I'm not disagreeing, in lieu of a better theory, you're probably right but I'd like to know how such a clean intake & crankcase cause the intake skirt & cylinder to score like that from dirt ingestion that is otherwise totally invisible. I'm going to give it another look - I think I missed something.
I don't believe it was straight gassed, what came out of the tank was... not straight gas; can't vouch for the ratio or what kind of oil he used but it was mixed.
I wonder if bad bearings could have that effect; if it's got that lateral play (it does) maybe the piston is rubbing too hard on the intake wall?

 

[Stihldoc]

Scoring on the intake side of the piston is a tough one. Depending on where on the skirt it is, and whether it is horizontal or vertical will help you figure it out.
Vertical scoring can be from dirt ingestion, as previously stated. Or.... it can be from water in the fuel. Water in the fuel is one we rarely check for unless we see murky fuel, but a little water that has been picked up from ethanol fuel, for example, will cause intake side scoring.
Horizontal wear- I said scoring earlier, but thats not accurate- is often the result of ice ingestion. Ice damage to a piston is usually a rounding-over of the bottom of the intake side piston skirt. Carburetor icing is an environmental condition and not a defect in the saw. Carburetors naturally run 20-30 degrees colder than the air temperature because of the chilling caused by the low air pressure past the venturi. This is why the saw has "winter" pre-heat door near the air box. Its possible to have a carburetor ice up when the outside temperature is in the 50's. The ice crystals are very abrasive inside the motor and can cause lots of strange damage.
Without pix of the intake side of the piston, its hard to diagnose this failure.

 

[Hunnry]

Scoring on the intake side of the piston is a tough one. Depending on where on the skirt it is, and whether it is horizontal or vertical will help you figure it out.
Vertical scoring can be from dirt ingestion, as previously stated. Or.... it can be from water in the fuel. Water in the fuel is one we rarely check for unless we see murky fuel, but a little water that has been picked up from ethanol fuel, for example, will cause intake side scoring.
Horizontal wear- I said scoring earlier, but thats not accurate- is often the result of ice ingestion. Ice damage to a piston is usually a rounding-over of the bottom of the intake side piston skirt. Carburetor icing is an environmental condition and not a defect in the saw. Carburetors naturally run 20-30 degrees colder than the air temperature because of the chilling caused by the low air pressure past the venturi. This is why the saw has "winter" pre-heat door near the air box. Its possible to have a carburetor ice up when the outside temperature is in the 50's. The ice crystals are very abrasive inside the motor and can cause lots of strange damage.
Without pix of the intake side of the piston, its hard to diagnose this failure.

Does the 271 have problems? I've read similar things on other forums.

 

[Mike Kunte]

Stated complaint: Customer brought in non-running saw, stated it ran fine until it died mid cut. Would not restart. Customer claims it is a carburetor issue.

Initial visual exam and test run: Unit looks to be clean, well cared for. Engine pulls over fine; piston is not frozen, compression feels a bit low. Will not start following correct starting procedure, also won't start w/ WOT.

Diagnosis: Following Stihl engine check sheet the full diagnosis reveals a few interesting facts about the unit:

• The saw has approximately 110 lb of compression
• The intake side of the p&c are scored across the skirt
• Rings are free, exhaust side looks okay
• Unit fails vacuum (negative pressure) test, but passes positive vacuum test except:
• Bad bearings in unit mean lateral play in crankshaft is possible & will release all held pressure
• Which means the seals (primarily on clutch side) are bad
• Intact limiter caps on Carburetor rules out tampering; carburetor throttle shaft okay, carb passes positive pressure test; unit failure conditions not in line w/ bad carburetor

Analysis: The chain on the unit tightened to the point of being nearly impossible to turn by hand w/ brake released. Clutch sprocket somewhat worn. Likely cause of failure seems to be chain tightness pulling laterally on ball bearings and causing failure and introducing secondary air leak leaning out mixture until the scoring was bad enough to halt the unit. The problem was probably not as apparent as general seal failure (surging, poor idle & other typical issues). Unit died mid cut when it could no longer generate enough +/- pressure to operate fuel pump & transfer ports.

Conclusion: The saw is toast; the cost to repair the unit borders on $400 making repair economically unwise.

Wow! What a thorough and systematic approach! I have learnt so much from this thread. Thank you for sharing!!!

Mike

 

[stihltech]

Dirt came in through the failed seal. Seal failed due to chain being dull, stretching, then being overtightened because "it kept getting loose." Pretty common especially for a first time operator or one that has no clue what a sharp chain is.

 

[Mike Kunte]

Stihltech, good point!
I was taught by one of South Africa's top chainsaw men, Marius Taljard, how to tension a cold chain - cold being the operative word. When he first showed me how tight it should be, I thought he was joking. However, since doing it this way, my chains run perfectly, and only ever loosen up ever so slightly. I never have to re-tension them for the day. Having said that, I do keep the cutters very sharp, and never cut dull.

It's the guys who keep tensioning the chain who do the damage, which occurs when the now over-tight chain cools down and hurts the seals. In some cases the force is sufficient to destroy the bearings too.
Thanks for the great info!
Mike

 

[ValleyForge]

Dirt came in through the failed seal. Seal failed due to chain being dull, stretching, then being overtightened because "it kept getting loose." Pretty common especially for a first time operator or one that has no clue what a sharp chain is.

I’ve had a few 271‘s come in for repair where the HD2 air filter was lose on the mount. They were never locked in it place. All had brown burned pistons…

 

[TreeHuggerMasochist]

I was taught by one of South Africa's top chainsaw men, Marius Taljard, how to tension a cold chain - cold being the operative word. When he first showed me how tight it should be, I thought he was joking. However, since doing it this way, my chains run perfectly, and only ever loosen up ever so slightly. I never have to re-tension them for the day. Having said that, I do keep the cutters very sharp, and never cut dull.

It's the guys who keep tensioning the chain who do the damage, which occurs when the now over-tight chain cools down and hurts the seals. In some cases the force is sufficient to destroy the bearings too.

You have alerted me to a problem I didn't know I have. I beg of you to give details pertaining to correct chain tightness hot vs cold

 

[Sportfury70]

Just so happens I’m selling a 271 for half the repair cost. Tell your guy and I’ll ship it to you.

 

[Mike Kunte]

You have alerted me to a problem I didn't know I have. I beg of you to give details pertaining to correct chain tightness hot vs cold

Hey, THM!

There are several threads and videos on this topic, so this should not be taken as the ultimate guide to tensioning. Anyway, when starting up for the day, Marius showed me to tension the chain so that it "snaps back smartly" to the bar. Of course it's possible to over-tignten the chain, which can lead to premature stretching of the chain, and damage to the nose sprocket. So start off by going just a bit tighter than you normally do, and run the saw. Within a few minutes the chain will warm up and stretch, but it should still hang very close to the bottom of the bar.

If it droops, and you have to retension, then you should start off (cold) a bit tighter. Work your way up, and pretty soon you'll have the tension perfect.

In general, our Winter temps are not as low as those in the States, so if you're working in sub-zero temps, you may see more stretch, and may have to re-tension during the session. Of course, it's then critical to slacken the chain off before the saw cools all the way down to prevent damage to the bearings and seals. This is why it's so great to get the tension adjusted right, so that you don't have to re-tension.

Hope this helps! Let us know how it goes. That way we all learn.

Mike

 

[TreeHuggerMasochist]

Hey, THM!

There are several threads and videos on this topic, so this should not be taken as the ultimate guide to tensioning. Anyway, when starting up for the day, Marius showed me to tension the chain so that it "snaps back smartly" to the bar. Of course it's possible to over-tignten the chain, which can lead to premature stretching of the chain, and damage to the nose sprocket. So start off by going just a bit tighter than you normally do, and run the saw. Within a few minutes the chain will warm up and stretch, but it should still hang very close to the bottom of the bar.

If it droops, and you have to retension, then you should start off (cold) a bit tighter. Work your way up, and pretty soon you'll have the tension perfect.

In general, our Winter temps are not as low as those in the States, so if you're working in sub-zero temps, you may see more stretch, and may have to re-tension during the session. Of course, it's then critical to slacken the chain off before the saw cools all the way down to prevent damage to the bearings and seals. This is why it's so great to get the tension adjusted right, so that you don't have to re-tension.

Hope this helps! Let us know how it goes. That way we all learn.

Mike

Thanks MK! Fixing up my two saws now that have worn crank bearings, so this advice will certainly stick with me

 

[lone wolf]

Ensure that there is fuel in the tank and that it is not stale or contaminated. If the fuel is old or contaminated, drain the tank and refill it with fresh fuel.
look at the spark plug to make sure it is clean and properly gapped. If it is fouled or damaged, replace it with a new one.
Check the air filter to make sure it is clean and free from debris. If it is dirty or clogged, clean or replace it Check the carburetor to make sure it is clean and functioning properly. If it is dirty or damaged, clean or replace it.

What!

 

[lone wolf]

Stated complaint: Customer brought in non-running saw, stated it ran fine until it died mid cut. Would not restart. Customer claims it is a carburetor issue.

Initial visual exam and test run: Unit looks to be clean, well cared for. Engine pulls over fine; piston is not frozen, compression feels a bit low. Will not start following correct starting procedure, also won't start w/ WOT.

Diagnosis: Following Stihl engine check sheet the full diagnosis reveals a few interesting facts about the unit:

• The saw has approximately 110 lb of compression
• The intake side of the p&c are scored across the skirt
• Rings are free, exhaust side looks okay
• Unit fails vacuum (negative pressure) test, but passes positive vacuum test except:
• Bad bearings in unit mean lateral play in crankshaft is possible & will release all held pressure
• Which means the seals (primarily on clutch side) are bad
• Intact limiter caps on Carburetor rules out tampering; carburetor throttle shaft okay, carb passes positive pressure test; unit failure conditions not in line w/ bad carburetor

Analysis: The chain on the unit tightened to the point of being nearly impossible to turn by hand w/ brake released. Clutch sprocket somewhat worn. Likely cause of failure seems to be chain tightness pulling laterally on ball bearings and causing failure and introducing secondary air leak leaning out mixture until the scoring was bad enough to halt the unit. The problem was probably not as apparent as general seal failure (surging, poor idle & other typical issues). Unit died mid cut when it could no longer generate enough +/- pressure to operate fuel pump & transfer ports.

Conclusion: The saw is toast; the cost to repair the unit borders on $400 making repair economically unwise.

Well, that's what happens when they overtighten the hell out of the chain, I suppose! Let that be an expensive lesson.

 

[KostasVolos]

There's another possibility also when it comes to scoring at at the intake side of p&c .
Water ( rain particularly ) ingestion.
Usually scoring is located from rings and downward on the skirt.
Can happen also when too many times or too much ether ( starting fluid ) is used to start the saw.


( There's probably a mistake on the manual : replace "alcohol or either" with
"alcohol or ether" .)

 

[cscltd]

If chain was super tight and fails leak down. Could be pto crank bearing failed and cage broke up and is making lean like score marks on piston.
I had a 291 with exact issue. But piston was ok on this one. Just needed new bearings. (It ran but vibrated like it was missing 4 flywheel fins on one side- could see the crank wobble when reved up

 

[stihltech]

Just to clear up a statement- I believe you meant the machine marks were visible across the piston. not up and down scoring?