MS 661c cylinder issue

[seancpr1985]

Hello all, I am new to site so thank you in advance for any help or advice. I have a bunch of equipment I work on so I'm sure I will be posting and helping as much as I can. Ok...onto my current issue.
I have a MS 661c with the M-tronic carb. It is used as a trencher rather than a saw. They use it to dig irrigation lines in the town's parks. This is the third time I am having this issue and would like some insight on it if possible. The first time it broke down it was being used for a while no problem and then out of no where the rings got smoked. I was not there at the time, I dont work on the road, I just repair the trucks and equipment. but I was told it ran out of fuel and they added fuel to it and it was MIXED and not straight gas, so they say.... they started it back up and that is when it destroyed the cylinder. I replaced cylinder/piston parts from a kit (STIHL PARTS). So now its back in service again. different crew this time. Using saw and all of a sudden it locked up, could only pull recoil part way and it would stop moving. took it apart and found that the crankshaft was destroyed. Threw the saw away and bought another brand new MS661 C. Been used a few times no problem, ran out of fuel, added fuel again, ran for 5 minutes...rings smoked. I as told once again that straight fuel was not added. What the heck is going on!?!? Could it be that when it runs out of fuel it is causing an extreme lean condition and loss of lubrication and that is what is destroying these cylinders? I guess my question is, does it damage the saw to run out of fuel often? Or I guess they could be lying and pouring straight gas in, blowing it up and then topping it off with mixed fuel so I can't tell. I just don't think that 3 different groups of grown men could all do the same thing, but anything is possible. What are your thoughts or ideas to keep this from happening again. About to replace cylinder again.

Also I am an automotive mechanic by profession, and for the last few years I have been fixing small engines like concrete saws, pumps, tampers, cement mixers and bobcats and stuff like that for a municipality. I understand engines and the operations of most things so feel free to get as technical as you want. I will most likely be able to understand. Thank you and if more info is needed just let me know and I will update as needed.

 

[huskihl]

I doubt the saw has enough load on it to bring the rpm down. Excessive rpm isn’t really good for any saw, but particularly the 661.

 

[Vintage Engine Repairs]

If it was a new saw that was used a few times, why didn’t you take it back to the Stihl dealer?

 

[seancpr1985]

I doubt the saw has enough load on it to bring the rpm down. Excessive rpm isn’t really good for any saw, but particularly the 661.

Thanks for replying. As far as the high RPMs are you referring to when it is about to run out of fuel? The mean condition causes excessively high rpm? And the saw dogs through the earth up to 18” deep. I’ve never actually used it but I would think it would put a decent load on the saw. Maybe not though.

 

[seancpr1985]

If it was a new saw that was used a few times, why didn’t you take it back to the Stihl dealer?

It doesn’t get used daily. So it’s probably been over a year since we purchased it. Maybe they could help. I didn’t really think of that. Hopefully the fracture we use it for digging and not for cutting doesn’t void any warranty. The trenching system which replaces the bar was purchased separately.

 

[huskihl]

Thanks for replying. As far as the high RPMs are you referring to when it is about to run out of fuel? The mean condition causes excessively high rpm? And the saw dogs through the earth up to 18” deep. I’ve never actually used it but I would think it would put a decent load on the saw. Maybe not though.

Not necessarily when running out of fuel, but in general use. I’d be curious to watch one day and see how high it’s turning

 

[BradSt]

Do you have any pics of the carnage? Was the crankshaft/crankcase dry? You can usually tell a straight gassed 2 stroke, because the crank/bearings/crankcase won't show any residual oil. If you find dyed oil in there, it probably wasn't straight gassed.

 

[seancpr1985]

Not necessarily when running out of fuel, but in general use. I’d be curious to watch one day and see how high it’s turning

I do start it at the shop before I send it out. To make sure it starts and to show the calibration procedure. It did seem kind of high, but it was going up and down as it calibrated it self. After it was done I released the throttle and it idled down fine. I’m guessing I should probably get some sort or RPM measuring device. What should it be running for RPM, and what is considered too high?

 

[huskihl]

I do start it at the shop before I send it out. To make sure it starts and to show the calibration procedure. It did seem kind of high, but it was going up and down as it calibrated it self. After it was done I released the throttle and it idled down fine. I’m guessing I should probably get some sort or RPM measuring device. What should it be running for RPM, and what is considered too high?

If it spends much time over 13k its not going to be happy. Same as running all day long at 12k

 

[RED-85-Z51]

In general its bad to run a 2 stroke out of gas, not just because of the extremely lean condition as it runs out but they will tend to pull some air leading up to that point.

Sent from my SM-G996U using Tapatalk

 

[seancpr1985]

If it spends much time over 13k its not going to be happy. Same as running all day long at 12k

What tool do you use/recommend to measure RPM?

 

[seancpr1985]

Do you have any pics of the carnage? Was the crankshaft/crankcase dry? You can usually tell a straight gassed 2 stroke, because the crank/bearings/crankcase won't show any residual oil. If you find dyed oil in there, it probably wasn't straight gassed.

Here are some pics of the latest carnage. Unfortunately I don’t have any of the 2 previous issues. It was a while back. There is definitely some oil on crank as you can see. There wasn’t much on the roller bearing however. Not sure if that is supposed to be oil or greased. Maybe not. Not sure how oil would get up there. What do you think after seeing pics?

 

[seancpr1985]

This is what the complete unit looks like. If this helps at all. It sits in that yellow cradle next to it.

 

[seancpr1985]

In general its bad to run a 2 stroke out of gas, not just because of the extremely lean condition as it runs out but they will tend to pull some air leading up to that point.

Sent from my SM-G996U using Tapatalk

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.

 

[Yukon Stihl]

What mix ratio are you running?
Please don’t throw out dead saws ,there are always peoples that will dispose of them properly. I need a dead 661 for parts for my 661.

And that goes for the scored cylinders too.
Does the crew actually have straight gas with them? If they do human error is probably the most likely

 

[seancpr1985]

What mix ratio are you running?
Please don’t throw out dead saws ,there are always peoples that will dispose of them properly. I need a dead 661 for parts for my 661.

And that goes for the scored cylinders too.
Does the crew actually have straight gas with them? If they do human error is probably the most likely

Yes I realize that now. And I will keep you in mind when this one inevitably blows up again. Sorry about that. The crews don’t have either in their service trucks. However if they take a saw they take a can of mix that I make my self. 50:1 is the ratio. We use Sten Mix for oil. And the gas out of the pumps here is “mid grade”. Not sure if exact octane rating. If they have a cement mixer or a tamper with them they will also take a can of straight gas. But they are completely different in size. Mix is 2gal labeled MIX and straight is 5gal not labeled. Human error is still possible I suppose though.

BTW...great profile pic.

 

[toadman]

Hmmm, possibly part throttle use?
even the M-tronic doesn't have a part throttle fuel circuit, and having used these before, it sounds like it's gonna explode at full throttle.
Most guys first response is to run less than full throttle, as it sounds easier on the saw.
This is a terribly hard application for a chainsaw.
the digging cutters don't really load the motor properly, and it relies on the operator to do everything perfectly to keep the saw happy, which almost never happens.

 

[toadman]

We swapped all our trenchers like this over to 3.5 briggs edger motors, I had a lot of trouble keeping the saw heads alive.
Never had a problem with the same saws in wood...

 

[seancpr1985]

Hmmm, possibly part throttle use?
even the M-tronic doesn't have a part throttle fuel circuit, and having used these before, it sounds like it's gonna explode at full throttle.
Most guys first response is to run less than full throttle, as it sounds easier on the saw.
This is a terribly hard application for a chainsaw.
the digging cutters don't really load the motor properly, and it relies on the operator to do everything perfectly to keep the saw happy, which almost never happens.

So you are saying maybe the best course of action should be to keep in under full throttle unless absolutely necessary? I’m sure part throttle would still get the job done as long as they aren’t trying to rush through it. The calibration procedure does require it to be at full throttle for almost a minute. Which I have done myself, and it does sound a little crazy.

 

[seancpr1985]

We swapped all our trenchers like this over to 3.5 briggs edger motors, I had a lot of trouble keeping the saw heads alive.
Never had a problem with the same saws in wood...

I’ll have to look into this. Is there something you could send me or post so I can see exactly what you means and are running?