Stihl MS200 - cause and effect - what caused this?

[MartDalb]

Hey everyone

Got a Stihl MS200 today. (No info on story or user etc)

Wondering what caused this piston to look like this.

Pictures to help identify the cause.

1. Piston exhaust side
2. Intake side
3. top
4. Muffler


Comments are most welcomed!

 

[MartDalb]

pressure tested, no leaks.
fuel in tank seemed fine, clear fresh etc.

Dirt from muffler caused this?

 

[J D]

Is there a film of oil inside the crankcase?
What was the carb set at?

 

[MartDalb]

Is there a film of oil inside the crankcase?
What was the carb set at?

No film inside crankcase, and crank/shaft looks perfectly fine, as do bearings.

Carb was H limited to 3/4 turn and L was exactly 1 turn, so no sign of issues there.


oh and on a very positive note, the cylinder is very easy to clean up and should be perfectly fine to reuse.

 

[J D]

There should be a light oily film inside the crankcase... if its dry I'd say that's a pretty good indication it was straight gassed. 3/4 of a turn out on the high isn't necessarily enough either so that could have played a part too, as would being covered in gunk if the saw was.
Hopefully you get away with just a piston & rings. Wouldn't be a bad idea to put new seals in it anyway, they're cheap enough. Be sure to vac/pressure test on reassembly & rotate the motor while doing so

 

[MartDalb]

There should be a light oily film inside the crankcase... if its dry I'd say that's a pretty good indication it was straight gassed. 3/4 of a turn out on the high isn't necessarily enough either so that could have played a part too, as would being covered in gunk if the saw was.
Hopefully you get away with just a piston & rings. Wouldn't be a bad idea to put new seals in it anyway, they're cheap enough. Be sure to vac/pressure test on reassembly & rotate the motor while doing so

Agree, the inside seem dry, too dry.

Plan was to find a piston and while at it, a set of oil seals, just to be sure.

Taking these things apart takes some time, might as well do seals while at it.

Thanks for the quick replies

 

[lone wolf]

Vac leak or lean tune did it that saw needs to be vac and pressure tested again. The impulse line or air boot could leak too, did you look at them closely? Is the H on the carb at 1 turn out with the limiter off? Test it again vac and pressure and look at the intake boot and impulse line closely. The carb is also suspect. Clean and replace the parts in the carb and take the H limiter off and set at 1 turn out. When it's running use a tach to set the rpms at 14K with bar and chain on and the saw should idle evenly, not jump around a lot.

 

[MartDalb]

Vac leak or lean tune did it that saw needs to by vac and pressure tested.

Tested the seals, vac and pressure, showed nothing - I will be replacing seals tho. Why not.

Can't say if it was lean tune or not, carb wasn't set incorrectly (when I got it, can't speak for how it was run of course).

Perhaps it's a case of, someone putting in strait gas, running it, and then putting in mixed, realizing the mistake?

Or fiddling with the carb, running it lean and the workshop who had it, reset the carb, before handing it to me.

I like these mysteries, part of the fun of making things work again, for me anyways.

 

[lone wolf]

Tested the seals, vac and pressure, showed nothing - I will be replacing seals tho. Why not.

Can't say if it was lean tune or not, carb wasn't set incorrectly (when I got it, can't speak for how it was run of course).

Perhaps it's a case of, someone putting in strait gas, running it, and then putting in mixed, realizing the mistake?

Or fiddling with the carb, running it lean and the workshop who had it, reset the carb, before handing it to me.

I like these mysteries, part of the fun of making things work again, for me anyways.

Straight gas would score the piston on both intake and exhaust sides. Dunk it in water to really test good for leaks. How's the intake boot look is it cracked?

 

[lone wolf]

https://www.madsens1.com/how-to-read-a-damaged-piston

 

[MartDalb]

Straight gas would score the piston on both intake and exhaust sides. Dunk it in water to really test good for leaks. How's the intake boot look is it cracked?

I had it pressured up nicely, with soaby water on both sides. - nothing showed at all. It held pressure and vac.

From the link you post, the overheating, lean, seems to be spot on.

edit. And intake boot is intact, no sign of damage etc.

 

[MartDalb]

Rechecking the engine...It seems really, really dry. Not a spot of lubrication to be seen.

the crankshaft and bearings turn fine, but the entire thing just really screams lube....

Had a few hedge trimmers apart just yesterday, and the difference is nite and day in the engine/lubrication part.

Perhaps user ran it strait gassed for a few seconds, handed it over to repair shop, who then put in mixed gas, to test/check the saw?

That would make some kind of sense, although, signs should show on intake side, right.

 

[grizz55chev]

I had it pressured up nicely, with soaby water on both sides. - nothing showed at all. It held pressure and vac.

From the link you post, the overheating, lean, seems to be spot on.

edit. And intake boot is intact, no sign of damage etc.

Wolf is giving you the best advice you could ask for, listen carefully and follow every step! He's done more work on this one particular model than anyone you'll likely ever come accross. Only thing I'll add is don't use aftermarket parts, maybe a Meteor piston if you can save the cylinder, but that's the only exception!

 

[MartDalb]

Wolf is giving you the best advice you could ask for, listen carefully and follow every step! He's done more work on this one particular model than anyone you'll likely ever come accross. Only thing I'll add is don't use aftermarket parts, maybe a Meteor piston if you can save the cylinder, but that's the only exception!

noted thanks.

And I will follow the advice given.
Not the first time @lone wolf helps me out here!

 

[lone wolf]

noted thanks.

And I will follow the advice given.
Not the first time @lone wolf helps me out here!

Rip the carb apart and inspect the parts that are affected by fuel.

 

[stihltech]

The saw ALWAYS has good fuel in it when it comes to the shop. Hard to get some people to own up.
You got great advice here. Go to it and give us an update.
Always remember- THE SUSPENSE IS KILLING US!

 

[lone wolf]

As long as it is 100 percent airtight and the carb is working good, and you tune the saw properly and make sure you have proper mix no need to worry when it's together again. Put new, piston, possibly cylinder, seals, gaskets, impulse line, fuel line and filter and rebuild the carb. Remove the limiter off the H jet and tune at 1 turn out also pull the screen out of the muffler.

 

[Lightning Performance]

As long as it is 100 percent airtight and the carb is working good, and you tune the saw properly and make sure you have proper mix no need to worry when it's together again. Put new, piston, possibly cylinder, seals, gaskets, impulse line, fuel line and filter and rebuild the carb. Remove the limiter off the H jet and tune at 1 turn out also pull the screen out of the muffler.

Are you still using those crap factory 200 seals?

 

[MartDalb]

As long as it is 100 percent airtight and the carb is working good, and you tune the saw properly and make sure you have proper mix no need to worry when it's together again. Put new, piston, possibly cylinder, seals, gaskets, impulse line, fuel line and filter and rebuild the carb. Remove the limiter off the H jet and tune at 1 turn out also pull the screen out of the muffler.

noted and will follow this, thanks!

 

[singinwoodwackr]

Sure looks like air leak to me.
I have a similar looking 192 and a Husky 340 in the shop.
The 192 ran and had signs of obvious air leak.