Sthil MS250 issues

[hunting09999]

I have a MS250 chainsaw, it has great compression and starts well and runs fine till you start too cut. After roughly 3 minutes it looses cutting power and bogs. Take it out of the wood and throttle it it sounds fine, back into the wood it bogs immediately. I’ve changed cleaned carb,changed to new (sthil) carb, checked tank vent line and cleaned tank vent line filter, changed plug to Bosch only plug,used sthil canned gas gas,checked screen in exhaust for blockage. This one has me baffled.
Any help to fix this would be greatly appreciated

Thanks

 

[hunting09999]

I have a MS250 chainsaw, it has great compression and starts well and runs fine till you start too cut. After roughly 3 minutes it looses cutting power and bogs. Take it out of the wood and throttle it it sounds fine, back into the wood it bogs immediately. I’ve changed cleaned carb,changed to new (sthil) carb, checked tank vent line and cleaned tank vent line filter, changed plug to Bosch only plug,used sthil canned gas gas,checked screen in exhaust for blockage. This one has me baffled.
Any help to fix this would be greatly appreciated

Thanks

O,and changed fuel tank pickup filter also

 

[g-man57]

Sounds like it may need a carb adjustment. Maybe too rich at full throttle?

 

[ILENGINE]

Sounds like you have about covered everything. Don't rule out a faulty ignition module. don't know if that module has spark advance but if it does it could be going haywired when warm. Haven't noticed it much on Stihl, But have had a couple cases of it on the larger saws. but have had to replace several modules over the years on Makita/Dolmar, Solo, due to that exact symptom.

 

[hunting09999]

Sounds like it may need a carb adjustment. Maybe too rich at full throttle?

Ok I messed with adjustment all ready but I will try that specially and see if I see any change, thanks

 

[hunting09999]

Ok I messed with adjustment all ready but I will try that specially and see if I see any change, thanks

Thanks too g-man57 it was a very fine high speed carburetor adjustment that got it to stop bogging. Still have to keep it screaming to cut but I ll mess with it alittle more and see what happens

Thanks again gman

 

[g-man57]

Thanks too g-man57 it was a very fine high speed carburetor adjustment that got it to stop bogging. Still have to keep it screaming to cut but I ll mess with it alittle more and see what happens

Thanks again gman

Glad to help. It's why we're all here. Still screaming to cut? Just a thought... I think we all mix our gas too rich - fear of damaging the unit. Try a tank of pre-mixed fuel and see what happens... I that works, measure your mix very carefully going forward.

 

[Fish]

Look closely at the fuel line, espesially where it bends going onto the carb, they are well known for cracking. Also check and make sure that the impulse line is still attached to both nipples.

 

[Forest#2]

One thing to keep a heads up about.
Just a tip to be aware of in your description of how the saw is acting.
This can happen fast and ruin your Stihl.
I've seen Stihl saws do what you describe, mainly due to the H jet being adjusted too LEAN, but can be other things also.
Saw cuts good if just rimming and on/off the throttle but place the saw in a long cut for more than 30 seconds to 1 min or longer load and the saw starts bogging and rpms drooping, take the saw out of the cut, rev few times and it picks up speed, back into the cut and it acts like log is pinching the saw or saw dust clogging the chain. Lift out of cut and rev again and back into the cut. More times it goes back into the cut the worse it gets.
You have to be careful because if it's overheating due to usually leaning out or cooling fins clogged the piston skirt is swelling into the cylinder and the saw is eating itself. Can happen fast. This is why the rpms are dropping in the cut, piston friction against the cylinder.
You need to pull the muffler and look at the piston.
When tuning my saws I use a IR thermometer and when in a full bar cut I monitor the block temp for engine overheating. (easy to do by yourself) Just use a 3/4 to full bar cut and watch the temp as the saw loads up, if gets to 350 fast and is going above your are in the too hot zone and piston is starting to swell usually and the saw will eventually start losing rpms due to piston friction) Anything over 350 is not good. A 1/4 turn in sometimes on the H jet on a Stihl is too lean. I like to adjust for the 4 cycle sound out of the cut which tells me I'm on the rich side of the carb adjustment. I rarely use a tachometer.
If your piston is already scored but the saw starts and runs good you have the option of replacing the piston and jug or adjusting the carb correctly and use as is.
I've seen guys install new piston and jugs and ruin the new parts again due to incorrect carb adjusting.