Stihl MS440 Bad Compression

[Skinnynnif]

First time poster but I've read a few threads and seen the common questions so below is my novel of what's going on and I've done.

Bought this saw used, don't know much on this history but it was running good last year. I've never owned a big saw so it may have been down on power but it felt like a beast compared to the tiny little echo I had run before. Towards the end of the year it didn't like to idle but the saw is now 800m (2600ft) higher in elevation to where it used to live so i chalked it up to needing some carb tuning. Had the same issue this year so I had a buddy help me dial the carb in and it was running great..... until the next week when it wouldn't start at all. Gave up and brought it to a shop and we checked the compression and it reads 90. Tester has a valve at the bottom and a short hose and we pulled and pulled until the needle stopped climbing.

The threads on the spark plug were looking rough but it did thread and seat really nicely and we were able to get it tight, so it may not have many spark plug changes in it, but I don't suspect it is leaking. It has the decomp delete in it but I cant get a wrench past the plastics to pull it and check the crush washer on the back (that will happen this weekend). Took off the exhaust to check the piston and it looks good to me and the intake side of the cylinder doesn't show anything to my eye either. I can also see the rings move (very slight, not loose) as the piston moves which tells me they aren't seized. (photos below)

I'm not opposed to doing a meteor piston and head, I just want to make sure I'm not spending money for the wrong thing; it would fix the spark plug issue and any eliminate any questions about the head. I could start with just rings as a little diagnosis but it will need a top end at some point anyways.

I'm hoping the knowledge of the people here can help guide me on some other things to check. I cut a few chords of wood for heat in my house plus some fire wood while camping so I don't want to spend a bunch of money chasing issues. I'm also comfortable taking apart small engines and tinkering, but buying a case splitter for 1 seal feels like a stretch. If someone here is in SE BC and wants to help out over a case of beer, I would also be in favor of that!

Plan to check next is:
- decomp delete
- intake boot (although this shouldn't kill compression)
- Head gasket
- piston ring gap (need to find feeler gauges but could be done)

TLDR: Bad compression, piston looks ok, please help.

Thanks!
-Finny

 

[babaoriley]

I think new rings and gasket would fix this problem. Piston and cylinder look good to me and if it has low compression it gotta have a leak somewhere. Rings don’t seal all the way, gasket leaks that’s all I can think of. But what’s the cause for those things go bad I don’t know. Someone with more experience will chime in soon, hang tight!

 

[Mad Professor]

Do a pres/vac test then take off P/C to see what is in there.

 

[Indy Red]

I am currently rebuilding my 044 and my cylinder looked perfect but when I installed new rings the ring gap was .012/.013" which means although it was not damaged it was still worn and had it's best days behind it.

From what I can see those scratches do not look terrible but you will need to pull the cylinder to be sure, pressure test it first because you could end up burning up anything new you put on it. For it to be 90 that would indicate really worn rings/piston and or cylinder and I imagine would need replacement but get it pressure tested, I purchased a Mityvac kit, best purchase I have made in years.

Someone more experienced than me will be along to help i'm sure, helped me loads so far.

 

[ZeroJunk]

Any way you can test your compression gauge on a good running saw ?

 

[Tobystihl]

If the rings were worn, you would expect to see carbon (black staining) on the piston skirt.

 

[Gord404]

probly an air leak or maybe you high speed screw was not adjusted right- too lean-vac - comp will tll you what is going on and go from there- I can get a good pressure positive sometimes but vac the needle drops- both are important

 

[muddy42]

Dont spend any money until you have pressure and vac tested it. This is different from a compression test (and I’d argue more important). Minor leaks are best diagnosed by doing a pressure test under water, this wont harm the saw.

Seriously this testis the mainstay of any diagnosis, all the potential problems leaks you list will be obvious in a matter of seconds with a line of bubbles.

 

[stihltech]

As stated above- diagnose first. DO NOT rely on the compression gauge!