Stihl MS250 mixture screws?

[ericm979]

It's not normal. Something happened to it. Could just be crud in the engine though that usually affects the intake side of the piston more.

You might have an air leak. That will throw off the fuel/air ratio much more at idle than at full throttle. But it will lean it out everywhere, and that might have been enough to cause a partial seizure.

You might be able to see the piston crown through the exhaust port. (shine a light through the plug hole). That might tell you more about the piston's condition.

It's hard to take pics down in the port but if you can clear ones of the exhaust skirt, post 'em.

 

[nyone]

Can you post pics of the piston scoring? Some light scuffingbis ok but a compression test will tell the tale. At about 90psi I had a stihl 026 that would not start/run.

 

[mike48130]

Can you post pics of the piston scoring? Some light scuffingbis ok but a compression test will tell the tale. At about 90psi I had a stihl 026 that would not start/run.

You are usually more concerned with the wall of the cylinder. The piston can have some slight wear marks, but the cylinder wall can not have any scoring. Usually I find when the walls are scored, it runs worse when hot or won't start when hot.

 

[ericm979]

It's true that the cylinder walls are more of a concern and light scoring on the piston might not be a problem. But the cylinder is not visible without disassembling the engine (or maybe using a bore scope), while you can see the piston skirt through the port.

 

[mike48130]

It's true that the cylinder walls are more of a concern and light scoring on the piston might not be a problem. But the cylinder is not visible without disassembling the engine (or maybe using a bore scope), while you can see the piston skirt through the port.

You should be able to see one side of the cylinder by taking the muffler off. It will give you an idea of how clean it is.

 

[ericm979]

True, but there's a lot of problems that can happen where the upper part of the intake side of the cylinder does not show damage. If it's bad enough to damage that part then it's pretty bad. The OP's problem does not sound that severe.

Here's a good guide to reading piston damage in saws: Piston Failure Analysis

 

[dodge man]

It bothers me some because as I drug a screw driver lightly across the piston I thought I could feel the score. I will try and post a pic.

 

[dodge man]

The first pic is more the top of the piston the second more the bottom. Tough to get good pics.

 

[ericm979]

That looks bad enough that I'd rebuild it. When you do at least replace the piston, rings, wrist pin bearing and circlips. The cylinder may be in good shape but check for scoring and deposits of aluminium. I haven't rebuilt a clamshell type saw but I think the crank seals are easy to get out when you pull the crank out of the cylinder, so replace them too. They may have been the problem that caused the damage and replacing them is cheap insurance.
I have the shop manual pdf if you need it.

 

[dfkrug]

The first pic is more the top of the piston the second more the bottom.

Sorry, Dave, but your piston and probably cylinder are trashed. Good photos.

I have had good experiences with aftermarket P&C for the MS250. You can get a set for $20 from a US seller.

If your pre-mix fuel is kept fresh and at the right ratio, an air leak is a good suspect, as Eric suggests. You also may have over-revved it or worked it too hard in hot weather.