MS 661c cylinder issue

[huskihl]

It’s hard to see from looking at your scoring pictures. Usually, excess RPMs will cause the piston skirt to break or it will rattle the ring locating pin loose and allow the ring to turn and catch in a port. Yours actually looks like seizure from excess air or poor lubrication.
On an oil test done on a different site, Stens FD rated oil came in dead last out of about 25 oils tested. I’m not saying that’s it, but it certainly wouldn’t hurt to switch brands

 

[BradSt]

That looks like a lean seize to me (I've burned up my share of 2 strokes)...burned on the exhaust side only. A lack of lube failure should have scoring all the way around. Also, those counterweights have plenty of lube on them. I have to admit, I've never seen one of those contraptions before. Ask the guys how they are running the throttle...If they are part throttling it under heavy load, that may be your issue. At least at full throttle, the Mtronic is in full control of fueling, and will cut spark to keep it below its rev limit, which will actually cool the piston.

Side note, my parents have one of those Stihl FS56 motor'd cultivator. It is designed to run WOT, up against the rev limiter. Drives me nuts listening to it. I keep it loaded enough to minimize it, but Stihl says that's how they run. Sounds like it has a different ignition module than the trimmer, because it limits so low.

Could attach a "tiny tach", to see what the RPM is actually at, but any trained ear should be able to hear the rev limiter coming in (guessing around 13k ish?). A few bumps here and there are totally fine, but I don't like the idea of sitting against it for minutes on end.

 

[seancpr1985]

It’s hard to see from looking at your scoring pictures. Usually, excess RPMs will cause the piston skirt to break or it will rattle the ring locating pin loose and allow the ring to turn and catch in a port. Yours actually looks like seizure from excess air or poor lubrication.
On an oil test done on a different site, Stens FD rated oil came in dead last out of about 25 oils tested. I’m not saying that’s it, but it certainly wouldn’t hurt to switch brands

Well that is some useful information to know about the Stens. Thank you for that. And looking back on all the cylinder/piston kits I’ve done on all our Stihls and even one husqvarna, there is one constant, and that is the Stens oil!! What kind of oil do you run/recommend? What ratio, 50:1? 40;1?

also you said it’s hard to tell from the pics I posted. Is there other picture angles or whatever that I could post that would help you help me? I still have all the parts. Just took it apart today. Thanks.

 

[seancpr1985]

That looks like a lean seize to me (I've burned up my share of 2 strokes)...burned on the exhaust side only. A lack of lube failure should have scoring all the way around. Also, those counterweights have plenty of lube on them. I have to admit, I've never seen one of those contraptions before. Ask the guys how they are running the throttle...If they are part throttling it under heavy load, that may be your issue. At least at full throttle, the Mtronic is in full control of fueling, and will cut spark to keep it below its rev limit, which will actually cool the piston.

Side note, my parents have one of those Stihl FS56 motor'd cultivator. It is designed to run WOT, up against the rev limiter. Drives me nuts listening to it. I keep it loaded enough to minimize it, but Stihl says that's how they run. Sounds like it has a different ignition module than the trimmer, because it limits so low.

Could attach a "tiny tach", to see what the RPM is actually at, but any trained ear should be able to hear the rev limiter coming in (guessing around 13k ish?). A few bumps here and there are totally fine, but I don't like the idea of sitting against it for minutes on end.

Ok thanks I will poke around and see if I can find info on here or maybe another category to post in and find out how they are being run.
And I’m. Not to familiar with the M-tronic at all since this is the only machine we have with it, so maybe I should get some learning materials on that to get a better understanding. I’ll try and grab one of those tiny tacos and see what it’s putting out when back together. I wouldn’t say I have a trained ear, but at the same time I’m. Not completely oblivious. So they gauge will help some. And I can be sure and train my ears. I’m trying to see if one of the guys has the video of me showing them how to calibrate it this last time. Maybe some of your trained ears could have some mor insight.

 

[huskihl]

Could also run a larger drive sprocket to bring the rpm down a little. But it looks like there’s an air leak that the mtronic can’t compensate for or the mtronic just isn’t putting out enough fuel.
What’s the number on the coil? I’m guessing it’s running the white solenoid?

 

[seancpr1985]

Could also run a larger drive sprocket to bring the rpm down a little. But it looks like there’s an air leak that the mtronic can’t compensate for or the mtronic just isn’t putting out enough fuel.
What’s the number on the coil? I’m guessing it’s running the white solenoid?

I don’t have the machine in front of me anymore, I’ve left the shop for the day. But when I get in tomorrow morning I’ll take a peak at the coil and solenoid and let you know what I find.

 

[RED-85-Z51]

Yes I’ve had similar issues with our TS410/420’s cut off saws. And our TS 700’s has also smoked a few cylinders. Maybe it does have to do with running out of fuel then. I just don’t see how that could be so destructive. But I see what you mean about pulling in the excess air easing the RPM. Makes sense. I’m never there when they blow up so it’s hard to say what is exactly happening during its final breath.

Lean means less fuel, less fuel means less oil...less fuel also means it burns hotter.

Hotter, less oil...recipe for disaster

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[RED-85-Z51]

Looks straight gassed to me, thats catastrophic failure

 

[seancpr1985]

Looks straight gassed to me, thats catastrophic failure

Used for maybe 30 feet of trenching they said. Ran out of fuel. Put more fuel in and it started up and conked out. Couldn’t get it going again. That’s the story anyways.....

 

[RED-85-Z51]

Usually when people mess up they try to cover their butts, like when a mower comes in with a rod slung out and just full of fresh oil and they have nooo idea what happened

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[J D]

Sounds like your power heads have pretty much everything working against them. If you are set on using chainsaws to drive the ditcher a different model may be better suited. Something that doesn't scream so much & has appropriate power to run at WOT. I'd want to be running it at 40:1 or even 32:1 with a good synthetic oil & tuned rich at about 500RPM below max. I don't think m-tronic is going to do you any favors here either.
Couple of other things to consider:
How long does the gas sit around for... Long enough for the oil to settle?
When you rebuilt the last one did you pressure/vac test it?
I would suspect your crank bearing failure wasn't a second failure as much as a further result of the initial lean &/or low oil condition... just my 2c, let us know how you get on

 

[seancpr1985]

Sounds like your power heads have pretty much everything working against them. If you are set on using chainsaws to drive the ditcher a different model may be better suited. Something that doesn't scream so much & has appropriate power to run at WOT. I'd want to be running it at 40:1 or even 32:1 with a good synthetic oil & tuned rich at about 500RPM below max. I don't think m-tronic is going to do you any favors here either.
Couple of other things to consider:
How long does the gas sit around for... Long enough for the oil to settle?
When you rebuilt the last one did you pressure/vac test it?
I would suspect your crank bearing failure wasn't a second failure as much as a further result of the initial lean &/or low oil condition... just my 2c, let us know how you get on

This power head is the one that came with the trencher initially and is what they recommended. I believe now they use a MS462? Not sure about that. It’s Australian company called “terra trencher”. Cost about $4500 complete. Maybe more. The second time I rebuilt it, I did not test it. And then the crank failed and we got rid of it. This one that has currently failed is a brand new power head that I had never taken apart. The fuel had sat for a bout a week or 2 since last use.

 

[TBS]

Are you putting oil in the bar oil tank when you run this attachment?

 

[J D]

If it's that new I'd be taking it back under warranty.
All the afore mentioned issues are going to be accentuated running a new saw that's still breaking in.
After 2 weeks, if the fuel can didn't get a bit of a shake before the saw was fueled there's a good chance the first tank was pretty light on oil

 

[seancpr1985]

Are you putting oil in the bar oil tank when run this attachment?

No it says to not use bar oil at all. It says to use a aerosol chain and cable spray on the attachment chain only, every tank of fuel. Although the time the crank got smoked the person using it had added bar oil I believe. Not entirely sure if it was that time but at one time I did realize someone added bar oil and I removed it.

 

[seancpr1985]

If it's that new I'd be taking it back under warranty.
All the afore mentioned issues are going to be accentuated running a new saw that's still breaking in.
After 2 weeks, if the fuel can didn't get a bit of a shake before the saw was fueled there's a good chance the first tank was pretty light on oil

Yeah I’m. It sure about the tank. They used. We have so many. Could have been sitting a while. And the fuel itself was sitting in the machine for a week or 2 also. As far as warranty goes I mean I’m not worried about the money because it isn’t mine. And I was told to take it apart and repair it. So that’s what I do. But if it was mine you better believe I would be bringing it back.

 

[TBS]

No it says to not use bar oil at all. It says to use a aerosol chain and cable spray on the attachment chain only, every tank of fuel. Although the time the crank got smoked the person using it had added bar oil I believe. Not entirely sure if it was that time but at one time I did realize someone added bar oil and I removed it.

The oil pump on the power head needs to have oil so it doesn't overheat. If it gets to hot the crank seal will fail. the Mtronic system can compensate for air leaks until they get too big.

 

[RED-85-Z51]

The oil pump on the power head needs to have oil so it doesn't overheat. If it gets to hot the crank seal will fail. the Mtronic system can compensate for air leaks until they get too big.

Im not sure this attachment uses an oil pump

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[RED-85-Z51]

The oil pump on the power head needs to have oil so it doesn't overheat. If it gets to hot the crank seal will fail. the Mtronic system can compensate for air leaks until they get too big.

Im not sure this attachment uses an oil pump

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[TBS]

Im not sure this attachment uses an oil pump

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From the parts i see on their website it uses the bar oil pump to provide lubrication for the drive chain since the bar adapters are slotted for the oiler. You'd still need oil even if the attachment didn't because you'd kill the oil pump unless you took the pump out.