Stihl ms250 bogging at full throttle

[SaltyTimber]

Hey all. I'm having some trouble with my Stihl ms250. It idles fine, and typically runs well, but last time I used it I had cut for about and hour and then it started bogging down at full throttle. And idea why this would happen? Thanks!

 

[Ozhoo]

In addition to this list, check to make sure that the spark arrestor is clear.

 

[Khntr85]

If you saw had sat for any length of time the carb could need cleaned....also make sure fuel lines have no cracks...

 

[Chainsaw Jim]

Put a brand new spark plug in it and run the saw without the air filter to check those two components. If problems persist, pull the muffler off and have a peek at the piston. If it looks good then check the muffler for any obstruction or dirty spark screen when you put it back on. It doesn't hurt to check the flywheel coil gap and give the crank a check for any sloppiness from a bad bearing.
I move on to the carb from there. Remove it and when disconnecting the fuel line the fuel usually squirts out if the fuel cap isn't loosened. If it squirts out that means the pick up filter is good.
The tank vent will likely need replaced because it sits right next to the intake and sees a lot of fines. And you should probably buy a carb kit before you start because if you get that far it'll be time to replace it.

 

[SaltyTimber]

Thanks for the advice. I'll be tearing into it this weekend.

 

[oakchop]

I have the same issue. My Sthil MS 250 three times now has ran perfect for an hour and then it begins to bog on throttling or bog when trying to begin a cut and then you have to stop. Its not my filter. Its not my fuel. I also religiously clean the carb filter and make sure no debris get in the carburetor.
What is the runtime on MS 250's? Does anyone know? It seems to run great evertime cutting a tree down, then ill get to the last cut or cuts and suddenly its starts bogging when attempting to cut a log or branch and the saws no good till tomorrow. Next day itll run perfect for another hour felling a tree, cutting it up and then begins this bogging down issue a hour in. Im having a feeling its the motor getting too hot or perhaps running the saw too long which is why im askinig what the runtime is on this MS 250 saw. I want to say I read this somewhere before and that its runtime was under 2 hours and 4 hours max or something?
I shouldve got a farm boss. In fact I am going to get one.

 

[cookies]

I ran into this recently...new impulse hose, tank vent, complete carb kit, plug and fuel filter, readjust carb ad she was a ripper again

 

[oakchop]

Yea my next try was getting a Zama carburetor made in PHIL (not china version) and try that. I think maybe the check valve in the carburetor shows a gumming up issue that produces about a hour into runtime. This manifests each time i run my saw for a hour of cutting perfectly. on the exact time, a hour in, the saw suddenly just bogs and wont cut. itll bog out and almost die at attempting any cuts. This has happened on my last 3 oak tree fells for firewood. In fact on my last 2 cuts left the saw wont cut.
I also know that MS 250 is the most troublesome saw from all forums, videos and stories I hear, PLUS my own experience.
some things i will do now is dumping my fuel out and running the saw till it dies to keep the fuel from gumming up the carburetor (although ive been using non-ethanol 93 octane 50:1 truefuel mix) if the saw isnt used for a few days. I know you do this if your storing it for weeks but for days? seems overkill but ill try it. anything to keep this MS 250 running for more than a HOUR! sheesh. shouldve bought a farm boss or something way stronger.

 

[cookies]

Yea my next try was getting a Zama carburetor made in PHIL (not china version) and try that. I think maybe the check valve in the carburetor shows a gumming up issue that produces about a hour into runtime. This manifests each time i run my saw for a hour of cutting perfectly. on the exact time, a hour in, the saw suddenly just bogs and wont cut. itll bog out and almost die at attempting any cuts. This has happened on my last 3 oak tree fells for firewood. In fact on my last 2 cuts left the saw wont cut.
I also know that MS 250 is the most troublesome saw from all forums, videos and stories I hear, PLUS my own experience.
some things i will do now is dumping my fuel out and running the saw till it dies to keep the fuel from gumming up the carburetor (although ive been using non-ethanol 93 octane 50:1 truefuel mix) if the saw isnt used for a few days. I know you do this if your storing it for weeks but for days? seems overkill but ill try it. anything to keep this MS 250 running for more than a HOUR! sheesh. shouldve bought a farm boss or something way stronger.

The check valve deals with the idle circuit if memory serves, I would suspect the impulse hose, over time they reform to the barb on the cylinder base loosing their seal once fully heated up. Without a perfect seal the pressure/vac pulse grows weaker esp at max rpm where vacuum is less than at idle.

 

[oakchop]

The check valve deals with the idle circuit if memory serves, I would suspect the impulse hose, over time they reform to the barb on the cylinder base loosing their seal once fully heated up. Without a perfect seal the pressure/vac pulse grows weaker esp at max rpm where vacuum is less than at idle.

Yes funny you say this. I just watched a YouTube video on this. I guess the black hose falls off this orange nipple but you can't see it. it's like under/between your chainsaw. Only way to check it is getting 2 screws off under the throttle handle and one screw off the side to separate the saw a little to check it. reattaching it is another job in itself. long needle nose pliers without serrations would help.
This could be my issue but then again, why does my saw start up and run perfect everytime and only begins bogging after 1 hour of a tree falling and multiple cuts. This air hose that assists in air pumping with gas mixing would be happening all the time. so my issue is likely something happening after this 1 hour runtime. then it goes away and starts all over again.
it's driving me nuts. I have to cut firewood out of 6 dead oak trees from 40 to 50 feet high and this saw pulling this after 1 hour is just plain annoying. I've already put in a new filter, ngk spark plug, fuel filter, oil filter, oil pump works great, clutch is new, carburetor works great (until 1 hour of runtime), spark arrestor is clean, chain is brand new, bar is brandnew, chainsaw is brand new too. it's had less than 10 hours runtime.
Going to try a Walbro carburetor but don't know which part number to get. the cross reference chart for walbro has many part numbers for Sthil for the Ms 250.
anyways I'll check that hose. next im going to check the piston under the muffler. after this I'm getting a carburetor. if it works after that I'm selling it and getting a farm boss.

 

[cookies]

Yes funny you say this. I just watched a YouTube video on this. I guess the black hose falls off this orange nipple but you can't see it. it's like under/between your chainsaw. Only way to check it is getting 2 screws off under the throttle handle and one screw off the side to separate the saw a little to check it. reattaching it is another job in itself. long needle nose pliers without serrations would help.
This could be my issue but then again, why does my saw start up and run perfect everytime and only begins bogging after 1 hour of a tree falling and multiple cuts. This air hose that assists in air pumping with gas mixing would be happening all the time. so my issue is likely something happening after this 1 hour runtime. then it goes away and starts all over again.
it's driving me nuts. I have to cut firewood out of 6 dead oak trees from 40 to 50 feet high and this saw pulling this after 1 hour is just plain annoying. I've already put in a new filter, ngk spark plug, fuel filter, oil filter, oil pump works great, clutch is new, carburetor works great (until 1 hour of runtime), spark arrestor is clean, chain is brand new, bar is brandnew, chainsaw is brand new too. it's had less than 10 hours runtime.
Going to try a Walbro carburetor but don't know which part number to get. the cross reference chart for walbro has many part numbers for Sthil for the Ms 250.
anyways I'll check that hose. next im going to check the piston under the muffler. after this I'm getting a carburetor. if it works after that I'm selling it and getting a farm boss.

It starts happening once the impulse hose gets hot, it starts leaking because it is stretched from heat/age. I also see them fail after the saw has been flooded, the fuel or even water gets into the hose and when cranked it breaks the bond the old hose had causing the leak because the liquid does not compress. Run the saw until it starts running bad then pressure/vac test to verify your leak.
You will need long curved jaw hemostats to reach it and to install the new one you will spend some time taking half the saw apart to access it. grand tota of 7 bucks from the dealer for one

 

[oakchop]

It starts happening once the impulse hose gets hot, it starts leaking because it is stretched from heat/age. I also see them fail after the saw has been flooded, the fuel or even water gets into the hose and when cranked it breaks the bond the old hose had causing the leak because the liquid does not compress. Run the saw until it starts running bad then pressure/vac test to verify your leak.
You will need long curved jaw hemostats to reach it and to install the new one you will spend some time taking half the saw apart to access it. grand tota of 7 bucks from the dealer for one

Oh I see. Thank you for confirming this. Based on the passing checklist I've already gone through, this vac hose is the only thing I haven't checked and it is the exact symptom at 1 hour (engine reaches high temp) use. It does takes the engine time to get hot since I'm only cutting 1 or 2 cuts along with some fast limb cuts then I turn it off to pull debris or I'll let it run to cool it down. I repeat this routine and eventually the engine reaches this higher temp where even external parts feel a little hot. I do pay close attention and when its hot I let it idle for 3 or 4 minutes and turn it off for 15 minutes before running again. place it in a shady breeze way to help cool.
But the pulse line or vac impulse line (I forget the actual name) seems to be the issue.
If it's not that I'm replacing the carb with a WT286 (Walbro) carburetor. I read another MS 250 thread on here where this guy bought this carburetor on Amazon and his 250 runs great now.

https://www.amazon.com/gp/product/B06XWHHGHH?psc=1#aw-udpv3-customer-reviews_feature_div
Checked reviews and everyone is saying it fixed their MS 250 and for fraction of the cost. I've read that Sthil is actually a Walbro carburetor but Sthil outsourced their manufacturing end to China and Philippines. Walbros design of how the carburetor works Ive read many times is far better because of spacing and other designs that eliminate certain issues ahead of time that otherwise arise in Sthil off the shelf saws. The facts in this are unclear but I have verified Walbro does have internals designed differently and they sell for half the cost as Sthil China or Philippines OEM.
In any case thank you for suggesting this overlooked area. Its worth checking before buying unnecessary parts at this point.

 

[oakchop]

Also forgot to say that this bogging thing happening at about an hour runtime also seems to happen especially around higher/extended rpm (trunk cuts) so it seems highly suspicious that this fits the impulse line leak theory even better than anything possibly going wrong in the carb. So will be checking this tomorrow and reporting back. I'll check some other things too.

 

[Okie]

AND if the piston is swelling into the cylinder wall due to the saw's block overheating it will bog, but the bog will eventually be a destroyed saw. This is usually caused due to the saw running lean. Saw will run fine for long time in light cuts, trimming, but put it too work on a log under load at full throttle and it starts bogging, lift it out of the cut and piss rev few times, it seems ok and back into the cut and bogs again after awhile.
Inspect the piston through the muffler port and if scored, you checked it too late.
This most generally is caused due to the OP setting the H jet too lean (too far in) to where the saw sounds really strong and smooth at full throttle.
Point being you can easily be destroying your saw's engine and not even realize such until it's too late.

A clogged spark arrestor screen will also cause a saw to operate ok for awhile, operating time varies with how restricted the screen has become.

 

[oakchop]

UPDATE: ok everyone I checked the pulse hose and it was connected so I checkced that off the list.
Next I decided to just take the carb out (Zama made in PHIL type) and boy Im gad I did. I discovered the tiny screen thats under the fuel pump was completely clogged with fine tiny wet paper like stuff! so much you could pull chunks out. I carefully removed the screen (yes its removalble and can be removed for cleaning despite claims to the contrary). I cleaned it with gasoline. cleaned out the inside with gas too. some of this debris you cant even see because its cream colored fibers of wood that look almost like milky paper.
put that together then removed the 2 screws to the diaphram. the diaphram hole was clogged too with a nice neat pile under that hole, basically blocking it. the diaphram looked and felt ok but gas sqirted out the seam of it when i gently pressed on it for a feel. is that normal? i pressed aroud the seal to close it, cleaned off the diaphram, the hole and put it all together. I didnt bother with the needle, bar and spring further under it.
Lets just say this carb was clogged so bad it was definately causing my bogging issue after it reached higher temps with a hour of cutting. likely the engine has suffered from this too in the form of damage. sad, the saw is brand new, literally less than 12 hours of runtime.
I odered 2 carbs with full kits as backups.
Why this is happening is likely due to me cutting down dead oak trees. They produce nice clean chips in the center trunks but the rest is dusty powder like sawdust and it doesnt help cutting close to the ground picking up more dust and wood thats falling as a fine powder.
Im going to test the saw now and report back.
I also discovered I cant remove my limiter caps. they seem to be a new type that you cant remove.

 

[oakchop]

UPDATE #2: after reajusting the idle screw the saw runs even better than before. response to throttle is lighting fast now and the rpm drop from throttle release is flawless. it even idles better. in a nutshell the carburetor was choking on crap. the fuel pump was blocked, the diaphram hole was plugged up.
this just goes to show, a plugged up Sthil WILL STILL RUN even for a hour but when heat builds up and you really get going it begins bogging. totally insane. whats crazy is this exact symptom is also related to the impulse hose being popped off.
Thank you guys for helping me. I was so frustrated.
However Im going to buy me a MS261 (with decompression valve) or a farmboss 271. both are 50cc. im leaning towards the 261.

 

[POGuy]

UPDATE: ok everyone I checked the pulse hose and it was connected so I checkced that off the list.
Next I decided to just take the carb out (Zama made in PHIL type) and boy Im gad I did. I discovered the tiny screen thats under the fuel pump was completely clogged with fine tiny wet paper like stuff! so much you could pull chunks out. I carefully removed the screen (yes its removalble and can be removed for cleaning despite claims to the contrary). I cleaned it with gasoline. cleaned out the inside with gas too. some of this debris you cant even see because its cream colored fibers of wood that look almost like milky paper.
put that together then removed the 2 screws to the diaphram. the diaphram hole was clogged too with a nice neat pile under that hole, basically blocking it. the diaphram looked and felt ok but gas sqirted out the seam of it when i gently pressed on it for a feel. is that normal? i pressed aroud the seal to close it, cleaned off the diaphram, the hole and put it all together. I didnt bother with the needle, bar and spring further under it.
Lets just say this carb was clogged so bad it was definately causing my bogging issue after it reached higher temps with a hour of cutting. likely the engine has suffered from this too in the form of damage. sad, the saw is brand new, literally less than 12 hours of runtime.
I odered 2 carbs with full kits as backups.
Why this is happening is likely due to me cutting down dead oak trees. They produce nice clean chips in the center trunks but the rest is dusty powder like sawdust and it doesnt help cutting close to the ground picking up more dust and wood thats falling as a fine powder.
Im going to test the saw now and report back.
I also discovered I cant remove my limiter caps. they seem to be a new type that you cant remove.

The MS 250 has the WORST air filter set up ever and will quickly bypass the filter as the little thin plastic box deforms from the restriction

 

[oakchop]

The MS 250 has the WORST air filter set up ever and will quickly bypass the filter as the little thin plastic box deforms from the restriction

I totally agree. I made my own 2nd inner filter that I place under it so all the crap stops bypassing the Stihl air filter and a lot does.
All you do is cut a diagonal piece that lays on the inside shelf of the black mold intake. then snap your air filter on. don't use anything that sheds particles or pieces from your cut or it'll suck right into the carb. you can use other chainsaw filters and just cut the dimensions. 1 1/8 x 3 1/8.
it's slightly wider than actual dimensions because when seating the filter you don't want it to grab and move the cut piece. I have mine perfect. it needs to be a filter rated for dirt. no micron rated stuff or it won't breathe.
you can even just make a oversized piece and place it over the intake and just push your filter into it, pinching the 2nd filter down, if it's thin enough.

 

[stihltech]

Glad you found the problem/ Full diagnosis requires visual inspection of all parts. Inside and out.
Zama carbs work just fine. None of them work if there is something clogged anywhere.
You found out the fuel system starts at is in the gas can and continues out the exhaust. Too many stop diagnosis part way. Always finish the journey.

 

[Okie]

I've seen what you describe inside chainsaw carbs that is so fine it gets past the in tank gas filter and clogs/restricts the screen in the carb. Saw usually idles and runs fine piss reving out of the cut but start bogging in the cut under load.

Most generally what I find is a very fine of what appears to be a mold of some sort attached to the walls of the fuel tank and very hard to remove. Almost like baking soda consistency and also I've seen different colors of the mold. Can be seen on fingers when a finger is wiped in the gas tank wall and sometimes can be seen with a good flashlight inspection.
I suspect it forms if a gas tank is left completely empty for long time and even worse if the gas tank is magnesium, galvanized, aluminum types and some plastics types. Plastic does not seem to be as bad for mold accumulation, but I've seen old gas turn to a thick skim like sticky molasses in plastic tanks and very hard to get completely clean. This usually happens over very long storage time and old gas turns to sticky type brown paste in the tank as it evaporates. The sticky stuff clogs all filters and fresh gas or mixed gas and vibration/aggitation just slowly dissolves the molasses for a come back issue.