Stihl MS250 bogs under constant throttle

[Den69RS96]

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.

 

[Bieber]

check the impulse line again, especially how it fits on the stud at the carb. if it's too lose it may blow air off...

a pressure test to check a bad seal or gasket would also be good if oyu can get the tools....

how does it idle? any changes in rpm if you tilt it when idling?

 

[TheManOfStihl]

How about half throttle, will it hold half throttle for awhile? I'm not thinking ignition since you can hold WOT, even for a short while.

Does the intake boot look good? Not cracked or broken? No debris between the carb and boot? I'm thinking the same as Bieber.
Fuel line is definitely not pinched anywhere?

 

[Den69RS96]

The intake boot and impulse line are in great condition. No cracks. The fuel line is not pinched Either. I checked those before I replaced anything. The saw idles fine. With the new carb I can move it around and no change in idle. It does the same thing at half throttle. When the throttle is held at at any position it begins to bog. I guess I'll pick up a new impulse hose and see if that helps.

 

[TheManOfStihl]

Do you have a vacuum/pressure pump to check the crankcase seals? I think that's what I would do next. It does sound like the fuel/air mix is the issue to me.

 

[AKDoug]

One of two things. Air leak or carb. What you describe is very similar to what I have found to be bad carbs on MS210/230/250 saws. I reach for a $30 WT215 carb and that's all it takes. Every once in a while I'll find an air leak to be the cause.

 

[z71mike]

I'm not a huge Zama fan. I've found them to be kinda finicky when I've used them and less likely to hold a good tune than my Walbros. I'd go Doug's route and acquire a Walbro.

 

[Den69RS96]

I have a vaccum pump and fittings that I can attach my air compressor. I'll have to make some plates that screw on. I'd rather rule out an air leak first since the carb on it now is brand new. Thanks for the help.

 

[TheManOfStihl]

I'm guessing you already know this, but in the interest of making sure you don't blow the seals, don't use compressed air from your compressor to pressurize the crankcase. You'll need to have a small hand operated pressure/vacuum pump with a gauge so you can easily see what the pressures are. The spec is 7.25 PSI of pressure must be held for 20 seconds to show the crankcase can hold pressure. The pull a vacuum of 7.25 in Hg.

If the vacuum reading remains
constant, or rises to no more than
0.3 bar (4.25 psi) within 20 seconds,
it can be assumed that the oil seals
are in good condition.
However, if the pressure continues
to rise (reduced vacuum in the
engine housing), the oil seals must
be replaced.

Don't exceed the listed pressures or you will damage the seals. Pardon me if you already know this info, I just don't want to see you take out your seals by over pressurizing.

 

[Den69RS96]

No I didn't have the specs for the testing. Thank you for the info.

 

[KG441c]

Ive used 10psi or less from my air compressor countless times with 0 seal failure from using compressed air?

 

[z71mike]

Obviously, if it's regulated.

 

[KG441c]

Right. If someone cant figure out they only need a small amount to test they probably shouldnt be working on the saw

 

[z71mike]

Agreed.

 

[TheManOfStihl]

I agreee guys I just want to make sure that he doesn't have a brain fart and blow his seals.

 

[HuskStihl]

What you describe is fairly standard MS250 performance

 

[AKDoug]

I've tuned MS250's with WT215's and a muffler mod (or old 025 muffler) that flat out rip.

 

[rich450es]

the ms250-025 is a great saw that hauls the mail with just the muffler mod and some fine tuning ....it is a very underated saw because it is a clamshell design compared to the "pro"designed ......we give this saw to the ditch diggers (guys first learning how to use a chain saw in the tree care company i work for ) because it is cheep to fix if they beat the s**t out of it or streight gas it

 

[Wood Doctor]

Could be just a clogged strainer in the carb that's robbing you of fuel at the top end RPM. Remove the carb and then the bottom screw to get to the strainer. Clean it with a toothpick and a toothbrush dipped in fuel. You might find a bunch of crud there, and if you do, the problem will likely be solved once that's cleaned out.

 

[Den69RS96]

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.