MS880 seized, again!

[NickJW66]

Hi all, just wondered if anyone out there has had the same problem.... Have Ms880 from new, this summer it siezed up. Taking advice from a repair technician  fitted new piston and cylinder, increased oil ratio mix to 40:1. Saw ran great since then, till yesterday when it nipped up, started again and then siezed solid. Exhaust side scored badly again, like a lack of lubrication. Technician now saying it could be leaking seal/gasket, will do pressure test...

.

 

[Stubby]

48 minutes ago, NickJW66 said:

Hi all, just wondered if anyone out there has had the same problem.... Have Ms880 from new, this summer it siezed up. Taking advice from a repair technician  fitted new piston and cylinder, increased oil ratio mix to 40:1. Saw ran great since then, till yesterday when it nipped up, started again and then siezed solid. Exhaust side scored badly again, like a lack of lubrication. Technician now saying it could be leaking seal/gasket, will do pressure test...

.

 

Air leak that was not fixed the first time me thinks .

[openspaceman]

1 hour ago, NickJW66 said:

till yesterday when it nipped up, started again and then siezed solid. Exhaust side scored badly again, like a lack of lubrication.

It is seldom lack of lubrication that causes a seizure on the exhaust side, more usually a problem from the exhaust being too hot, mostly from weak mixture.

 

When it nips up in this way the piston partially melts and some of the aluminium sticks to the bore. If you manage to start it again then the additional friction from this on top of the original mixture problem causes the final seize.

 

The other problem about a weak mixture is it also causes the saw to over rev, all these things combined are just too much for the oil film and the sliding parts.

[spudulike]

Pretty much every 880/088 I have brought back to life has been because it had been stored with half a tank of fuel for a few months and then used without refuelling. The oil in the fuel had been degraded over the storage period and using the saw again without fresh fuel was enough to seize it. Most owners were unaware that doing this was wrong and that the 2T oil was being degraded over time.

The other things that can do it...Air leak - not had this issue and the saw construction seems pretty good in this area, Carb High speed setting - I usually set this pretty safely just because these saws do very long cuts in the most extreme way with the longest bars so heat build up is much more likely. I believe there are two 880 carb types with 1000rpm difference on the high speed setting. The carb should be serviced to make sure it can deliver the correct fuel and a new air filter should be fitted. 

Other than this, the chain oiler arm on 880s have a tendency to slip around the big nylon gear and can result in owners thinking their saw isn't pulling correctly. I have done enough 880s to know this is a "go to" part to check on a service.

The rest of the saw is pretty bullet proof.

 

[bmp01]

On 15/12/2022 at 15:56, NickJW66 said:

Hi all, just wondered if anyone out there has had the same problem.... Have Ms880 from new, this summer it siezed up. Taking advice from a repair technician  fitted new piston and cylinder, increased oil ratio mix to 40:1. Saw ran great since then, till yesterday when it nipped up, started again and then siezed solid. Exhaust side scored badly again, like a lack of lubrication. Technician now saying it could be leaking seal/gasket, will do pressure test...

.

 

Ouch, thats not a great experience, painful even. I wonder if the "saw ran great" was too high max engine speed, was it 4 stroking out of the cut ? Difficult to say now I guess, unless you were in the habit of checking it routinely before it broke. 

Edited December 18, 2022 by bmp01
Made sense to me. Hopefully now, it makes sense to everyone else.

[john87]

On 15/12/2022 at 15:56, NickJW66 said:

Hi all, just wondered if anyone out there has had the same problem.... Have Ms880 from new, this summer it siezed up. Taking advice from a repair technician  fitted new piston and cylinder, increased oil ratio mix to 40:1. Saw ran great since then, till yesterday when it nipped up, started again and then siezed solid. Exhaust side scored badly again, like a lack of lubrication. Technician now saying it could be leaking seal/gasket, will do pressure test...

.

 

You say you started to put in extra oil.. This can cause the thing to run weak and overheat as more oil does TWO things. 1, weakens the mixture [as less petrol] and 2, lowers the octane of the fuel so the thing tends to pink..

Lack of lubrication would cause scoring all round the piston.. Scoreing just by the exhaust is overheating..

 

john..

[Stubby]

5 hours ago, john87 said:

You say you started to put in extra oil.. This can cause the thing to run weak and overheat as more oil does TWO things. 1, weakens the mixture [as less petrol] and 2, lowers the octane of the fuel so the thing tends to pink..

Lack of lubrication would cause scoring all round the piston.. Scoreing just by the exhaust is overheating..

 

john..

I have heard this theory and admittedly quoted it myself on occasion but I don't think The " weak as in less petrol " would cause it . I think its the weak in too much air that causes it to nip up . Too much oil will just soot up the plug and add carbon deposits .

[pleasant]

On 15/12/2022 at 17:36, openspaceman said:

It is seldom lack of lubrication that causes a seizure on the exhaust side, more usually a problem from the exhaust being too hot, mostly from weak mixture.

But isn't that a contradiction of terms? A weak mixture is a mix with less oil added, therefore less lubrication. Less lubrication equals more heat and a thinner protective oil film.

 

Like someone has already mentioned.......a leak down test should have been done to find an air leak before it was re-build last time. IMO

[openspaceman]

1 minute ago, pleasant said:

But isn't that a contradiction of terms? A weak mixture is a mix with less oil added, therefore less lubrication. Less lubrication equals more heat and a thinner protective oil film.

 

No I don't think so, the difference in the amount of fuel and oil is small. the significant difference, in the cut rather than over speeding, is the flame spreads more slowly so it is still burning as the piston is far down the stroke.To maximise thermodynamic efficiency the burning needs to take place near TDC so the expansion then pushes the piston down and this work extracts some of the heat,

 

If the mixture is still burning during this expansion this  means less work is extracted from the fuel and the exhaust becomes much hotter,

[peatff]

34 minutes ago, pleasant said:

But isn't that a contradiction of terms? A weak mixture is a mix with less oil added, therefore less lubrication. Less lubrication equals more heat and a thinner protective oil film.

 

That's the fuel to oil ratio, a weak mixture in usual terms is the carb adjustment air to fuel ratio running high revs with the H screw turned in.