Today's teardown 372xp, black crankcase

[singinwoodwackr]

So, pulled the cyl on the 372 the other day and the entire interior of the crankcase was lined with carbon, crank and all. A full teardown was in order, duh.
Question: what would cause this?
Saw belonged to a retired faller and his last time out was cutting fire damaged trees...saw had black crud all over it.
Piston had carbon on the exhaust side with just a small spot below the rings. Otherwise p&c are pristine.
Oil was seeping up through the case gasket under the muffler so I suspected an air leak in the case as well. Wouldn't hold vac and only held 5psi of pressure.

The crank cleaned up great in the sonic tank full of carb cleaner. Rod cage seems fine, no excessive play.

 

[Woodslasher]

Poor quality mix oil and possibly excessive heat too is my bet.

 

[singinwoodwackr]

Poor quality mix oil and possibly excessive heat too is my bet.

The chain oil was black and burnt smelling. Fuel was blue...no idea what mix it had. The saw ran great when I got it so I was just going to clean it up and run it. Glad I didnt...

 

[Paul Silvestri]

Soot is very very fine.. wouldn't be surprised if it made it's way in. I've seen some friends saws internals out here in Montana that cut in burn areas and they looked like that.

 

[huskihl]

Dirty air getting through the filter, ****** oil, plugged muffler will cause that.

The leak is common there. Added heat from the muffler dries and cracks the gasket. It’s not tied to the crankcase though. Holding pressure but not vacuum is likely the decomp sucking in under vacuum when you were testing

 

[spuldup]

You are a very brave individual! Kudos and good luck!

 

[singinwoodwackr]

Dirty air getting through the filter, ****** oil, plugged muffler will cause that.

The leak is common there. Added heat... from the muffler dries and cracks the gasket. It’s not tied to the crankcase though. Holding pressure but not vacuum is likely the decomp sucking in under vacuum when you were testing

Makes sense. I'll most likely plug the decomp...not needed

 

[singinwoodwackr]

You are a very brave individual! Kudos and good luck!

I have 8 saws in boxes for winter project assembly
I hope I can get them all back together, lol

 

[Gord404]

Bought 3 or 4 of those last year that were used for fire fighting and they all looked like that, my intentions were to take them down anyway and mod them.- complete saw with good cases and cylinders are gold to me, new bearings etc to start I do anyway.

 

[singinwoodwackr]

Dirty air getting through the filter, ****** oil, plugged muffler will cause that.

The leak is common there. Added heat from the muffler dries and cracks the gasket. It’s not tied to the crankcase though. Holding pressure but not vacuum is likely the decomp sucking in under vacuum when you were testing

Well, good call. Pulled the decomp valve and it is encrusted with carbon…no way it could ever seal. Plug installed
got the cylinder ports all polished today and into the 372 box it went for later assembly.

interesting thing. The bearings are in great shape after being tanked for a while. Nice and smooth. However, I’ll most likely replace them as the cost is small…they are 6202s and I can get quality ones at the local bearing shop.