Stihl MS250 bogs under constant throttle

[KG441c]

Get some rubber gasket material and cut a strip to block exhaust and I usually use another piece and block intake behind carb. And tap into the impulse line to vaccum or pressure check

 

[AKDoug]

Well, I did a vacuum test. I couldn't find any rubber stips in the house so I used duck tape. I duck taped the exhaust port and installed the muffler. I added a piece of duck tape to the intake boot as well. I attached my vacuum pump to the impluse hose and got it up to 7.5 on the vacuum gauge.. After 30 secs it dropped down to 7. According to a Stihl manual that is acceptable.

I'll have to find a hand held pressure pump to do a better pressure test. I set the regular on my air compressor to 10 psi and sprayed the crank seals with windex to look for bubbles. I held my finger on the intake boot to make sure the tape stayed on which it did and I saw no air bubbles. When I removed the air nozzle there was pressure in there, but I can't really tell if any was escaping elsewhere. I didn't hear anything but I'd rather check a gauge to be sure.

You don't need to do anymore. If it doesn't fail on vacuum you are fine. I only use pressure to find where the leak is after I check vacuum. Now it's back to the fuel system.

 

[KG441c]

Is it possible to hold vaccum but leak under pressure?

 

[AKDoug]

Yes. The seal lips can roll in and out and leak one way but not the other. However, when an engine runs the crank case forms a vacuum, so vacuum is more important.

 

[Rockjock]

A couple months ago I was using my MS250 to cut up oak branch that fell. The branch was maybe 8 inches in diameter. I just about finished cutting up the branch and put my saw on the ground to move the log for the final couple cuts. When i picked it up it died. It would not restart.

I took the saw into my workshop and checked it over. I removed the muffler. The cylinder and piston look brand new. The spark arrestor screen was clear, the fuel filter, fuel line, and impluse hose were all in good shape. I had trouble with the carb in the past so I put on a brand new Zama carb thinking that was my problem. I set it to factory specs. The saw started right up and revved nice. I put it away thinking it was all fixed.

A couple weeks ago, I needed this saw again so I fired it up. As soon as I put it into the cut it just about dies. I played around with the carb settings and it didn't help regardless which way i turned them. I figured maybe the fuel line was bad. So I replaced the fuel line and put on a new fuel filter. That didn't help either. I removed the tank vent and and it still dies in the cut. I put in a new spark plug and same results.

Finally it dawned on me to rev and hold the throttle steady and see what it does. So I revved the saw without putting it under load. If I pull on the throttle quickly and let off it revs up fine even if I do it mulitple times. However if I hold the throttle open, after a second or two in starts to bog.

Is this a tuning issue, maybe a bad coil? This saw has maybe 20 hrs total run time on it.

I have had something similar and it would up being the carb, a good cleaning for the 3rd time and it was running great

 

[Den69RS96]

Ok good to know. At least I ruled out an air leak. I'll put it all back to together. I originally thought the carb was the problem and thats why I bought a new one. I would hope a brand new carb wouldn't have any dirt or crude inside of it, but I'll take it apart and clean it anyway.

 

[KG441c]

Check the fuel tank strainer and also there is a strainer in the carb on the side the fuel line enters on

 

[AKDoug]

You said you bought a Zama. Where did it come from? I have very good luck with WT215's and they can be ordered through your local Stihl dealer for around $30.

It could still just be a tuning issue. Each screw should be 1 turn out from seated on most carbs.

 

[Den69RS96]

I bought it at my Stihl dealer. Once I get it back to together I'll give an update.

 

[Wood Doctor]

Check the fuel tank strainer and also there is a strainer in the carb on the side the fuel line enters on.

Ditto and see post #19 (mine).

 

[Bieber]

Ditto and see post #19 (mine).

I found out that the strainers are not a deal in most cases, only if you a ripped/faulty fuel filter in your tank.
Most of the problems are in the small passages of the carb body and I can tell cleaning with solvents and blowing out
with the airgun helps in just a few cases. In 3 of 4 cases taking an ultrasonic bath helps to solve this issue.

But his carb is new!? hm, though take apart and have a look.

 

[Bieber]

Do you have the flocked air filter version?

 

[Rockjock]

I found out that the strainers are not a deal in most cases, only if you a ripped/faulty fuel filter in your tank.
Most of the problems are in the small passages of the carb body and I can tell cleaning with solvents and blowing out
with the airgun helps in just a few cases. In 3 of 4 cases taking an ultrasonic bath helps to solve this issue.

But his carb is new!? hm, though take apart and have a look.

I am with you, checking out the carb will help eliminate any doubt I have had several MS250's and a few 025s that had the internal screen clogged up with gunk, a good cleaning and alles gut!

 

[Wood Doctor]

I am with you, checking out the carb will help eliminate any doubt I have had several MS250's and a few 025s that had the internal screen clogged up with gunk, a good cleaning and alles gut!

Well, as I look back on this thread, he said his carb was new. So, I guess the strainer is not the problem. I guess he should ship the saw to either you, me, or Bieber and be done with it. I might like to have this chain saw in my inventory (running, of course). However, it's a clam shell design, so I'm not so sure..

 

[VinceGU05]

Run it without the muffler and see if it runs WOT. I think blocked muffler or ignition probs.

 

[KG441c]

Could be a bad carb from factory

 

[Rockjock]

Well, as I look back on this thread, he said his carb was new. So, I guess the strainer is not the problem. I guess he should ship the saw to either you, me, or Bieber and be done with it. I might like to have this chain saw in my inventory (running, of course). However, it's a clam shell design, so I'm not so sure..

The 025/MS250 are a great little saw. Easy to sell day in day out. and work horses!

 

[Rockjock]

Could be a bad carb from factory

It is possible. Does not take much smutz to mess up a carb. Bogging down sounds like a fuel issue. Spark screen?

 

[KG441c]

Sounds more like a simple h screw adjustment to me

 

[CTYank]

The 025/MS250 are a great little saw. Easy to sell day in day out. and work horses!

Sorry, but not by my definition of "great". C'mon. The name is the selling point.