MS390 woes

[Unframed Dave]

Long story short, but I bought this of an ex "friend" as running and recently serviced. It ran for a while fine, but died abruptly. New plug and air fitted but still no good. 

 

This morning I put a compression tester on it and it struggled to make 70 psi. Popped the exhaust off and suspicions confirmed. The piston is fubard. 

 

The bore to my inexperienced eyes looks reasonable. 

 

Anyway, out of curiosity, I took my 250's exhaust off, to see if it was likely to be my fault. Bore and piston lovely and shiny. I've had this saw for well over a decade. 

 

Few questions:

 

1. Is it worth repairing the 390. I'm quite happy to have a crack at it myself? 

 

2. How did it manage to run for a fair while when I first acquired it? 

 

3. Should I just buy another and keep this one for spares? 

 

Thanks

Edited February 3, 2021 by Unframed Dave
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[Paddy1000111]

If it ran good for a while and then packed in are you sure you didn't accidently straight fuel it or ran it lean with poor H jet adjustment or a blocked fuel filter?

 

What's the rest of the saw like condition wise? Personally I would rather spend the money on a piston/cylinder kit and have a nice running saw than buy another which could pack in the same way. 

[Dan Maynard]

Problem might be that it had an air leak before your "friend" had it serviced, if they just replaced the piston without curing the leak then it is sure to fail again.

It is worth a meteor piston, they aren't usually expensive but really needs a pressure test as well.

[Unframed Dave]

2 hours ago, Paddy1000111 said:

If it ran good for a while and then packed in are you sure you didn't accidently straight fuel it or ran it lean with poor H jet adjustment or a blocked fuel filter?

 

What's the rest of the saw like condition wise? Personally I would rather spend the money on a piston/cylinder kit and have a nice running saw than buy another which could pack in the same way. 

Same fuel container as my 250, the 250 gets more use, so not fuel. The other two suggestions, I don't know, it ran perfectly well so unsure. 

Thanks for the suggestions. 

[Unframed Dave]

1 hour ago, Dan Maynard said:

Problem might be that it had an air leak before your "friend" had it serviced, if they just replaced the piston without curing the leak then it is sure to fail again.

It is worth a meteor piston, they aren't usually expensive but really needs a pressure test as well.

Thanks for that, how is a pressure test completed, I wouldn't know where to begin on that? 

 

Thanks for input. 

[spudulike]

I can see a smear of liquid gasket around the crankcase join. I am thinking that this may have had a new top end already and it is possible the seals weren't replaced causing another seize.

If the top end is anything other than OEM Stihl, don't just fit a new piston, fit a Hyway top end if you can get hold of one - a bit more expensive but better than most and have just done that on a MS250 which is for sale now!!

I would replace the seals and also check the impulse line and inlet boot for damage/splits.

The trouble with the uninitiated doing this sort of work is that simple issues can be over looked or not seen as issues if found. 

If you check the rubber parts are OK, you may get away without a leak down test but it is worth doing if you can - block the inlet, exhaust, impulse and then use an adapter on the plug hole to pressurize and vacuum the engine to make sure it holds pressure and vacuum.  

These machines are not that easy to work on due to their clam type engine.....just saying!

[Dan Maynard]

Thanks for that, how is a pressure test completed, I wouldn't know where to begin on that? 
 
Thanks for input. 

Ah spud rescued me there. It would mean buying the gear, and parts - or sending it out. Especially if it needs a cylinder as well maybe look it all up and balance that against how much you like the 390 anyway? Never going to be a world beating saw.

[Unframed Dave]

Thanks again for taking the time.

 

It sounds like it's probably not a job for me on my patio table then.

 

This may sound a bit daft, but this and the 250 are now my back up saws as I've gone over to battery. My only need for them is because I often spend a week or so in the woods working without electricity, so cannot charge batteries. The 250 is fine, and the 390 is only for heavier work (by my standards). Shame to dump though, as I have spare bar and chain for it. I'll probably investigate the cost of getting it sorted and take it from there.

 

Thanks again.

 

I'll pop a few pics of some of our recent work using the battery saws for reference. This is a hobby for us, both in our fifties (with the arthritis, etc that comes along) and love doing it.

 

 

[Unframed Dave]

All windblown trees, the one with the tape measure was 48 years old. Next winter, we will replant this area.