Please help with Stihl MS250

[Winchester356]

I had to replace the spark plug in my 250 toady is was not getting any fire. It cranks fine but runs kind of rough and lacks power. I am familiar with tuning saws but I could not get it to improve. I worked with it about an hour. Could this new spark plug be causing this?

 

[cedarshark]

Yes it could but it sounds like you had the same problem before you changed the plug. Are you sure you don't have a fuel delivery problem?

 

[Winchester356]

Yes it could but it sounds like you had the same problem before you changed the plug. Are you sure you don't have a fuel delivery problem?

Thanks for the reply it was running ok before the spark plug went out. It has been sitting for a couple weeks.

 

[dbus5sp]

I'm gonna second that, check your fuel system

 

[a. palmer jr.]

It's also possible to get a new plug that is bad. I've had a few of them, put an old plug in it and it runs just fine!

 

[tree_chomper]

Got any pics of old plug??

 

[drf255]

Got a picture of the old plug? Especially the electrode.

 

[jughead500]

For me champions have been notorious for being bad.
Sounds to me like your spark screen is plugged.

 

[a. palmer jr.]

I saw a picture online once of how spark plugs are made in China, not the ultra modern factories you'd think, more like my garage. Spark plugs were laying all over the dirty floor in a pile and not on an assembly line. This may not be true of all brands though..

 

[jughead500]

I saw where lowes had gotten so cheap that all they offer are some sort of cheaply made chinese plug.used to buy bosch back 2 years ago.then they sold out and only carried champion.sucks now I just about have to order plugs.

 

[a. palmer jr.]

I've bought plugs of differing brands from a local store and have had several that didn't work as well as the ones I replaced. I just have to buy several when I need one and do the shuffle to find a good one out of the bunch..

 

[cedarshark]

I have never had a bad bosch. OP...pull the plug, leave it in the boot, ground the threaded end against the engine and spin the starter rope. You should have a bright blue spark. If you dont, change the plug and try again. If the same, start eliminating your fuel problems. Filter,hose and carb. I assume you gapped the plug according to specs ?
Jughead made a good comment above...check to see if your spark arrester screen is plugged.

 

[drf255]

I have never had a bad bosch. OP...pull the plug, leave it in the boot, ground the threaded end against the engine and spin the starter rope. You should have a bright blue spark. If you dont, change the plug and try again. If the same, start eliminating your fuel problems. Filter,hose and carb. I assume you gapped the plug according to specs ?
Jughead made a good comment above...check to see if your spark arrester screen is plugged.

Just be careful about this. If the saw has fuel in it and you ground near the spark hole, prepare for a nice flame and some hair singeing. I learned this the hard way.

 

[Winchester356]

Thanks for all the help. I tried two other spark plugs it did the same with all of them. I even reved it up with the air filter off and it is still doing this. It is not the fuel either I am running it in my other saws.

 

[lone wolf]

Thanks for all the help. I tried two other spark plugs it did the same with all of them. I even reved it up with the air filter off and it is still doing this. It is not the fuel either I am running it in my other saws.

Let me understand the plug has no spark ,weak spark or it keeps fouling? Is the saw flooding?

 

[jughead500]

Spark arrester

 

[pantelis]

what colour is the spark ? open the gap between the electrodes then test.
test the spark plug boot inside have a spring , in what contition is this spring
how are you sure that is not something else ? lets say fuel problems
i have see saws today they pass all the tests , and after 3 days have leaks, or carb diaphragms problems. so give a look again
Good luck

 

[AKDoug]

Spark arrestor was mention above. Pull it and see if it's clogged. Since you now have the screws off, you might as well pull the muffler and check the piston condition. Once those two things check out, you need to move to the fuel and carburetor system.

 

[Winchester356]

I will check the spark arrestor tomorrow. I forgot to mention it is running very rich no matter how much I lean it out.

 

[lone wolf]

I will check the spark arrestor tomorrow. I forgot to mention it is running very rich no matter how much I lean it out.

That sounds like the carb has a problem.