STIHL MESSING WITH MY SAW ms250

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husqORbust

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Hey, I bought a second hand ms250 that at this point makes me want to rip my hair out...


The saw has it's original bar and chain on it, and the bar is mint. The plastics are mint, and the saw is visually in pretty good condition. I'm using those two indicators that the saw has some pretty low hours on it.


Before going to look at it I asked if it would hold its own weight on the starter cord and he responded with "150psi compression" or something pretty decent numbers wise. So... I figured I'll go take a look at the saw.


When I showed up, the saw looked pretty mint. Akin to a looking at a second hand car that had just been freshly detailed. I checked the gas tank... mint fuel in it, not a spec of dirt. Air cleaner was mint. Saw held it's own weight for awhile before slowly slipping through the compression and down to the next one...definitely didn't just yo-yo itself through the cord. I figured that ought to be pretty good.


Muffler was cool to the touch...so it didn't seem like it was sitting there hot waiting for some unsuspecting victim to show up and have it fire up half way thru the first pull...


After my preliminary inspections he asked if I wanted to hear it run and I go sure....he takes it from me and chokes it and pulls the cord twice. Puts it to high idle and yanks er twice more and it's flashed right up he clicked the throttle and it kicked down and idled perfectly. He revved er up a few times and handed me the saw.


I roll it all over the place, rev it up myself a few times, shut it off, flashes back up with half a pull, let it sit there and idle for a bit then roll it all over the place again and rev it up. Checked the oiler...works mint.


I'm thinking great...this saw should do exactly what I need it for. Go from cold start, flash right up in 5 pulls or less, buck down some trail blow down or buck up some wood to have a weekend fire or camping fire wood. Hand him some cash and go on my marry way.


I get home and couldn't help myself but to yard the muffler off and look at the piston and sleeve. Looks mint. The screen is mint too. I slide the muffler back on, torque it down and think I just made a great purchase.


2 weeks later I fire it in the back of my truck, go out and have a fire, and every so often it'd just stall out as soon as I finished a cut. Would fire right back up. I think that was kind of odd but oh well....carry on.


Couple weeks later next outing it would have slight bogging issue off idle and if I didn't run the trigger right to get it up to revs it would stall out....then it would stall out after a cut. Or just die randomly sitting there idling. Not really happy with the saw at this point but whatever, fire it back into the case and carry on.


Couple weeks later next outing I'd fire it up, and it would die as soon as I touched the throttle. So after about 5 start ups and dies I would finally get it to be able to rev up to full throttle...make some cuts, if I didn't keep it revving it would just die out...as soon as it died I'd have to mess with it for awhile getting it running...a series of pulling the trigger for a bunch of times killing it then I'd end up being able to get it back up to full throttle. Sometimes the saw will just run on high idle then die immediately when touching the throttle. Or, it'll die by itself after a few seconds of high idle.


Now...at this point... I'm done with this thing, my buddy's Husq has year old fuel mix in it, fires up 5th pull or less every time and is bucking wood down within 30 seconds of pulling it out of the case, cuts everything, runs mint, never messes around with it, and gets used about as frequently as mine sitting unused for weeks at a time... hands me the saw, I do whatever I need to do with it... clearly not a user issue. There's something wrong with my piece of crap Stihl.


I'm not a small engines tech, and before about 72 hours ago, I had no idea how a small engines carb worked, what an impulse line was, or had any knowledge of a chainsaw engine operation other than when I pull the cord I want it to start right now and buck trees in 30 seconds or less without ever messing with it...lol...wishful thinking.


Things I've checked, and where I'm at with the saw...

- Every time I went out, I mixed up a liter of 94 octane no ethanol and 50:1 with Stihl oil. Whatever didn't go in the saw that trip, got dumped out. Fresh mix every time. This is not an old fuel issue.

- I checked the plug...looked acceptable... replaced it anyways. Didn't solve any of my problems.

- Fuel filter is mint

- Tank breather is mint

- Air filter was probably not the issue...cleaned it anyways... it definitely isn't the issue now, and it being freshly cleaned solved none of my problems.


time to get a little more adventurous with this pile...


Yard the carb off. Stick a dab of grease in the impulse hole and pull the cord. Blows a hole clean through the grease. Going to say the impulse line is not my issue. Holding off on replacing...

I blow into the fuel line plugging the other end... holds lung pressure. I suck...holds lung vacuum...the fuel line is also nice and rubbery-pliabe-like and looks pretty mint and has no signs of decay. Holding off on replacing.
The intake rubber to block looks nice and premium too...Holding off on replacing that on a whim.


Now I'm thinking the problem is in the carb.............................................lol. Start googling to get the why for and how to on small engine carbs. Ya, this probably isn't so bad. Tear the carb apart...The pump diaphragm looked mint. The metering diaphragm felt mint too. All the ports looked free of obstruction...took a small rubber hose and placed them everywhere I could to blow and suck and they're all clear. I put the carb back together and try to simulate the way the metering diaphragm should work with lung pressure and all that..ya she's all good to go. The mechanical nature of the carb checks out.


Now, I'm googling the why for and how to for tuning the carb...thinkin maybe the carb tune is my issue and these limiter caps gotta go. Bin the limiter caps. Thread the needles all the way down and back the low needle out 1/4 turn and the high 3/4 turn..(I know now this isn't right, but bear with it)


Fire the carb back in the hole... the carb gasket is looking suspect at this point because half the stud hole is torn now but looks good otherwise as far as sealing the throat bore of the carb...figure should be good enough for a test hit on the new carb needle settings.

Take it outside, choke it yank the chord like it owes me money and it pops. Decent...move to half choke and yank the chord again like it still owes me money and the saw flashes right up into high idle and I click the throttle and it comes off high idle and for a second it idled then died...my crash course on google for carb tuning tells me the low speed is too lean...so I crank it out a quarter turn, put the saw back on high idle and yank the cord like it owes someone else money and I'm the enforcer...flashed up into high idle, I clicked er off high idle and it sat there and purred like it hasn't in a long time. I hit the throttle and it thought about revving up and dies. Flash it back up, turn low screw in till it wants to die then I thread it out till it wants to die then put it somewhere in the middle where it idled best...probably right around 1 1/4 turns out from seated. Turned down the idle screw so the chain stopped moving. Hit the throttle and it was like ya baby, super crisp and fast off idle response...would rev up till it ran out of fuel on the top end. Time to work on the high speed.


So I dial out the high speed and get it to the point where it's revving to the moon, and continue to dial er out til she's "4 stroking" and I'm happy with that. Maybe a tickle rich. Probably right around 1 1/2 to 1 3/4 turns out at this point.

She's idling like never before(except for the day I bought it... it was idling perfect...but I tuned it now, so it's idling better hahaha), roll tests mint, super crisp off idle response, revs up right now.....I haven't thrown a single part at this thing, and now she's running perfect. What the heck did even do other than take it apart a bit and put it back together.


Fire it back in the case thinking done deal...not even sure what I did but I'll take it. Later that day go grab my bud and we head er for the mountains. Pull it out of the case and it flashes up in 3 pulls or less, revs up a couple times, a little sloppy on the throttle response, dies out on idle... Now it's doing exactly what it was doing before.... Barely get it to rev up, dies, barely get it to idle, dies immediately off high idle. What a piece of junk. Good thing my buddy brought the Husq
 
Next day I try again with what I did...seat the needles, back them out...now this time it is completely unresponsive to any needle adjusting. Yank the carb off again, notice the outside of the metering valve diaphragm is wet. Hmmm... Go and get a carb kit... slap it in the hole...doesn't fix anything.



At this point I'm just inventing all kinds of new words and grasping at straws to try needle settings to get this thing to run. I end up taking some brake clean and shooting it directly into the carb throat and it flashes right up and dies.


OK, it HAS to be fuel delivery.


I yank the chord like it owes me so much money I should be able to buy a NEW FRIGGIN HUSQY and it sputters into high idle and I DOUCHE the outside of the carb in and out and all around with brake clean and it idles right up and I hit the throttle to take it off high idle and it's still idling with a slower chain but still spinning pretty good because I have the idle speed cranked right in and I back the idle speed down to a slight roar and at this point it's starting to putter out (was probably running on straight brake clean fumes at this point) so I frantically start backing out the low speed and I give it more turns than I can even count and then bam... It just sat there purring away. I dial back the idle screw to a non moving chain. I rev it up...super crisp throttle response. Runs out of fuel on the top end so I spin the high out for a nice gurgle on the top end.


OK, it HAS to be air flow at this point lol... It's sucking air from somewhere if it needs that much friggin' fuel.


Currently, the low speed needle is backed out 8 turns, and the high speed needle is backed out 2.5 turns. The saw fires right up in 2 pulls, 1 on choke one on...skipping high idle. Revs up like no ones business. Idles for days. Roll test doesn't change the idle. Douching it in and all around with brake clean maybe changes the idle a couple hundred rpm.


My intention is to throw as little money as possible at this saw, because it's condition is next to brand new. Having said that, can I rule out parts based on any of what I've done? I haven't vac or pressure tested the engine, replaced the impulse line or fuel line, or exhaust or intake gaskets... I don't know what it is. I'm thinking it's gotta be something pretty simple, but allowing a consistent amount of air in that isn't changing based on the saws position.

Questions:

- Can an exhaust gasket cause this?
- I plan on replacing the carb/intake gasket...it is a smidge suspect...simply because the carb has been on and off 10 times but it isn't torn around where the carb NEEDS to be sealed. Do they lose their sealing ability when they're wet/soaked/torqued down a bunch of times?
-Can I rule out crank seals? When it's running, it doesn't seem to be affected by roll testing. Can crank seals be tuned out, as I have done?
-Can I rule out fuel line? If there was a crack in the fuel line and sucking air, would it be able to be "tuned out", as I appear to have done or would it run suspect/hunting/leaning out/stumbling all the time no matter what?
-Can a blown head gasket be ruled out? Don't think it would run in any event if it were that??
-Can I rule out the impulse line? I kind of view this as a component that either works or doesn't.
-Would the plastic air box before the carb matter if it was slightly warped from over torquing?
-Can I rule out the carb being an issue? Can a carb that doesn't seal properly/function properly ever be tuned out?
Can the carb to block rubber intake boot be tuned out?

Well this turned out to be a pretty long post. If you made it this far, let me know what you figure. lol Watch me replace just the carb/intake gasket and solve all my problems.
 
Impulse hose (crack or loose/not connected) or fuel line (cracks in bend above the tank) ?? After extended non use the rubber parts tend to get brittle easily.

The impulse line goes from the crankcase to a connector under the air cleaner case. Loose or "just touching the connector" or small cracks will cause a slight air leak with the symptoms your saw shows.
Same goes for the fuel line. It may be tight at low vacuum (idling) and then let in air when the engine is revved.
Another possibility might be the crank seals.
 
I had the same problem with my ms250 ( now my sons saw) ran great first couple times after i got itnused then wouldnt hold an idle. Always messing with carb. Pressure tested and my crank seals were leaking. Replace and all good now.

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Impulse hose (crack or loose/not connected) or fuel line (cracks in bend above the tank) ?? After extended non use the rubber parts tend to get brittle easily.

The impulse line goes from the crankcase to a connector under the air cleaner case. Loose or "just touching the connector" or small cracks will cause a slight air leak with the symptoms your saw shows.
Same goes for the fuel line. It may be tight at low vacuum (idling) and then let in air when the engine is revved.
Another possibility might be the crank seals.

The impulse line is 100% connected firmly to the crank case and to the nipple. If it were disconnected, wouldn't that mean the carb doesn't pump fuel at all so I'd never get the saw running...
Cracked, Yeah... I can see how that would create a vacuum leak and operate the carb less efficiently requiring 1. more fuel for the air, but 2. requiring richer sittings to get fuel through the carb that is working less precisely. Didn't really think of it that way...

Cant see it being an intermittent fuel line issue... don't see it running as good as it does when it is running if the carb didn't see a steady supply of fuel. I view an air leak in the fuel line as air throughout the carb not being able to consistently deliver fuel, which it seems to do just fine as far as not sputtering/running erratically/hunting/etc. But..possibly I guess. Weirder stuff has happened before I'm sure.

Hopefully it's not crank seals.
 
Oh and welcome to the site
What part of canada you from?

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Dam. Little far for offerin help. Im in ontario.

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Sounds like the ignition is going out. It runs fine at first then starts cutting out randomly. It's an expensive part I know but if you have another saw like it you can try swapping it over and see what it does before you spend the money on a new one.
 
While you have the starter cover off check and see if the flywheel looks like this oneIMG_0171.jpgYou see that funny little nub just past the magneto??
A gentleman came in last year with a 250 that just didnt run right. I went to checking it and found that the magneto was spaced a significant distance from the flywheel. Then I found out why.... on the opposite side of the flywheel I find the offending casting flash. What I did was to file that sucker off FLAT, gapped the magneto correctly, and it ran and cut like a 250 should.
 
Thanks sb47 and Patrick...I'll check the flywheel and megneto out but I dont think it's ignition related.

The more I learn about 2 smokes the more I realize a pressure/vac test of the engine and a pressure test of the carb is the only way to diag if you dont want to randomly fire parts at it and cant clearly see a failed part.
 
While you have the starter cover off check and see if the flywheel looks like this oneView attachment 762987You see that funny little nub just past the magneto??
A gentleman came in last year with a 250 that just didnt run right. I went to checking it and found that the magneto was spaced a significant distance from the flywheel. Then I found out why.... on the opposite side of the flywheel I find the offending casting flash. What I did was to file that sucker off FLAT, gapped the magneto correctly, and it ran and cut like a 250 should.

I checked out my flyhweel and my saw has that nub on the flywheel, and is gapped appropriately to clear it. However, I 180'd the flywheel and the magneto is VERY close to the (timing marks? Not sure what theyre called) so, as is I could probably move the the magneto maybe a smidge... I dont know the specification but I'd say its super close! In any event that just means the flywheel wasnt machined on center...hopefully its balanced. Wishful thinking...

While I was there I figured I'd pop the impulse line off to get a better ruling on it. Impulse line is not cracked, and still feels nice and stiff/rubbery/pliable and is not brittle or gummy.

Will pop the fuel line out for a formal inspection...
 
Got my hands on a hand pressure pump/vac pump...
The base of the block does look a smidge wet/sweaty...possible that bar oil or fuel out of the exhaust can make it down and around there or is that a sign that the silicone is starting to let go? In any event.... it passed "spec" on the pressure test and vac.

pressured it and vac'd it through the impulse port on the block:
held 7.25 PSI for 20+ seconds
held 14" HG vac for 20+seconds. It could have dropped .2 bar and been acceptable allegedly but it didn't move.

Should I be spraying the seals and block and such with soapy water while pressure testing or is a simple " It holds 7.25 psi for 20 seconds " a good enough result?

Pressured up the carb to 11.5 PSI for 20 seconds or so. couldn't find a time duration Figured this was basically a pass or fail. Pass?
For S&G's I pressurized the impulse line to 12psi and dunked er under water - no leaks. Did the same with fuel line...no leaks.

Along with things I've already done:
Fresh fuel
Fresh plug
clear fuel filter
clear tank breather
clear air filter
Clear exhaust screen
Runs great with 8 turns of low and 2.5 turns of high...albeit slobbering fuel out the zawst.

Only thing that hasn't been accounted for at this point is the ignition(Doesn't explain why it needs so much fuel thru the carb needles?) and the carb/intake gasket and exhaust gasket.

Thing should run like no ones business with a fresh carb and exhaust gasket then, right? lol
 
EIGHT turns on the LOW screw ????:wtf:
That is enough fuel for a engine 4 times it's size.

Something is terribly wrong here. The 2 1/2 on the HIGH is atleast in the "ballpark". Eight turns on the LOW screw ?? And spewing unburned fuel out the exhaust...

Ok, Piston is going up and down in it's cylinder. Passes pressure and vacuum test. Carb needle and impulse are good.

Get a pointer down that cylinder and mark Top Dead Center on the flywheel. Let's make DAMN sure the ignition is firing at TDC, or even a smirch (like 1-2 mm) early.

I think the carburetor is doing what it is supposed to do. The crank seals are not blown out, but she just won't run without a TON of fuel.... puzzling.
 
Not sure what these results mean but I'll do my best explaining them.

Sitting behind the saw holding a screw driver with my right hand in the spark plug hole, left hand rotating the flywheel away from me, just as the screw driver stops moving up the tip of the magneto is about half way, or a smidge passed half on the first shiny point on the flywheel, then just as I feel the screw driver start to drop, the pointer is an 1/8th of an inch before the second shiny point.
 
Did you put a carb kit in when you cleaned carb?

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Yes, the carb has fresh diaphragms, gaskets, and a new metering needle.
I know what the issue is, almost put $ on it because it’s happened to me several times. You didn’t kit the carb correctly or the impulse hole going to the carb is blocked by a gasket. Take that carb apart download a decent diagram, make sure you are using an OEM kit, and trog away. Get all the screws really tight or it will suck air.
 
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