STIHL MESSING WITH MY SAW ms250

Okie294life

Okie294life

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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.
If you aren’t using the OEM kit or the wrong valves/diaphragms that come with the kit it will block the metering holes in the carb and it will not run.
 
Brent Nowell

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When u have the low screw out that many turns then you definitely have a vacuum leak or the metering diaphragm is not set correctly.
I’ve been saying this a lot but check the metering diaphragm lever and adjust it accordingly. A service manual will tell you, better yet a gauge for your carb will be more correct. Most walbros and zamas are set flush with carb body. But some are not. I don’t know anything about your saw but maybe someone here will chime up with the correct adjustment setting.
If that checks out, you have an air leak, probably a smaller one cause it doesn’t seem to affect the high nearly as bad.
 
Okie294life

Okie294life

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When u have the low screw out that many turns then you definitely have a vacuum leak or the metering diaphragm is not set correctly.
I’ve been saying this a lot but check the metering diaphragm lever and adjust it accordingly. A service manual will tell you, better yet a gauge for your carb will be more correct. Most walbros and zamas are set flush with carb body. But some are not. I don’t know anything about your saw but maybe someone here will chime up with the correct adjustment setting.
If that checks out, you have an air leak, probably a smaller one cause it doesn’t seem to affect the high nearly as bad.
I was going to say metering diaphragm. A lot of time there’s a spacer that goes under them, if it gets put on the wrong side or not at all it won’t pump gas.
 
Brent Nowell

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I was going to say metering diaphragm. A lot of time there’s a spacer that goes under them, if it gets put on the wrong side or not at all it won’t pump gas.
You might be mixing up the pump diaphragm as it has to be on the carb body with no gasket underneath due to the check valve functions. The metering diaphragm only controls the amount of fuel to the carb via the jets and other channels. Pump diaphragm pumps fuel to the metering diaphragm and is controlled via the impulse :)
 
Patrick62

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For lack of better description, but from the shiny points, I am very strongly leaning towards the flywheel spun on the crankshaft. Maybe the key was missing, or it sheared, or something. I believe the carburetor is doing exactly what it is supposed to do. WIth the adjustments way on the rich side, it is pumping fuel out the exhaust. Making little power at the same time... The intake and exhaust timing is set by the piston....

Remove the nut holding the flywheel on, put a screwdriver on opposite sides of the flywheel with a little tension. DO NOT BUGGER THE THEADS! but a square tap with a hammer (put the nut on flush with end of shaft) will usually release the flywheel.

There is a small key that indexes the flywheel to the shaft, if that was in the WRONG place, you have your answer.
 
husqORbust

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The carb kit is OEM and installed properly.

When u have the low screw out that many turns then you definitely have a vacuum leak or the metering diaphragm is not set correctly.
I’ve been saying this a lot but check the metering diaphragm lever and adjust it accordingly. A service manual will tell you, better yet a gauge for your carb will be more correct. Most walbros and zamas are set flush with carb body. But some are not. I don’t know anything about your saw but maybe someone here will chime up with the correct adjustment setting.
If that checks out, you have an air leak, probably a smaller one cause it doesn’t seem to affect the high nearly as bad.

I'll pull the carb back apart and check the metering lever.

It's a C1Q S242A carb. As far as I can tell based on pics of a Z gauge that would put the lever flush with the body.

For lack of better description, but from the shiny points, I am very strongly leaning towards the flywheel spun on the crankshaft. Maybe the key was missing, or it sheared, or something. I believe the carburetor is doing exactly what it is supposed to do. WIth the adjustments way on the rich side, it is pumping fuel out the exhaust. Making little power at the same time... The intake and exhaust timing is set by the piston....

Remove the nut holding the flywheel on, put a screwdriver on opposite sides of the flywheel with a little tension. DO NOT BUGGER THE THEADS! but a square tap with a hammer (put the nut on flush with end of shaft) will usually release the flywheel.

There is a small key that indexes the flywheel to the shaft, if that was in the WRONG place, you have your answer.

I'll pop the flywheel off later to double check. On a properly timed saw, where are the points on the megneto supposed to line up in relation to the S and N "shiny" magnet pads when the piston is at TDC?
 
Patrick62

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Without a set of old school breaker points to work with, this will be trickier. We used to be able to meter the opening of the points to coincide with the alignment of the magneto. You could fudge it a little, but the magneto generates the power at the same time that it uses it. I could see rigging up a timing light ( I have one that runs on D cells, that is nice to work with) and mark with orange paint one fin that delineates Top Dead Center. Might have to whirl the engine with a cordless drill as the starter would be in the way of seeing what was going on.

I think your carburetor is doing what it is designed to do. If i adjusted one of my saws 8 turns out on the L screw, it would be flooded and very quiet after that.
If there was a HUGE air leak from a bad seal, then it might run ok that way, but adjustin it would be a *****.
 
husqORbust

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Checked the metering lever... it was a brunette C hair lower than it was supposed to be... I adjusted it up anyways but it was honestly probably close enough before.

The flywheel key is 110% on the flywheel and in the crank groove.

I think I'll put it back together with a new carb/intake gasket and a new exhaust gasket and see if it'll run with a standard carb tune.
 
Patrick62

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Somethings just refuse to be "fixed"
I don't ever give up easy. Sometimes a wire could just be a 100,000 ohm resister until you PUT a meter on it... then magically it is a wire again!

I'd file that nubulous off the flywheel to be certain it ain't in the way and gap the Magneto with a business card (about .008 of a inch)
Just to be for darn sure that the ignition is getting all tthe magnetism it is supposed to get. Then, I would order a NEW ignition module, and try that.

At some point this little saw is gonna get up on the table and do a tap dance for us.
 
Okie294life

Okie294life

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Checked the metering lever... it was a brunette C hair lower than it was supposed to be... I adjusted it up anyways but it was honestly probably close enough before.

The flywheel key is 110% on the flywheel and in the crank groove.

I think I'll put it back together with a new carb/intake gasket and a new exhaust gasket and see if it'll run with a standard carb tune.
Get OEM for both the carb and gasket if at all possible. I just rebuilt a blower same issue, went the China way..you know, whole fuel system for 10$ shipped. It ran but was hard to start and would not rev all the way up. I just got back to OEM yesterday totally different deal.
 
husqORbust

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Well I ordered a new zawst gasket, intake gasket, and intake shim/sleeve whatever it is. Upon further inspection I noticed the plate was a bit tweaked out with the holes curvatured up and it didn't sit flat on the carb anymore... Didn't think it was always like that. Might have just been from when I torqued the carb down with my inner tube seal behind there during the pressure and vac test.

Should be about a week for those. Stay tuned I guess, lol

Somethings just refuse to be "fixed"
I don't ever give up easy. Sometimes a wire could just be a 100,000 ohm resister until you PUT a meter on it... then magically it is a wire again!

I'd file that nubulous off the flywheel to be certain it ain't in the way and gap the Magneto with a business card (about .008 of a inch)
Just to be for darn sure that the ignition is getting all tthe magnetism it is supposed to get. Then, I would order a NEW ignition module, and try that.

At some point this little saw is gonna get up on the table and do a tap dance for us.

I'm sure I'll get ambitious in the next week or so waiting for my parts..Ifnwhen I do I'll look up some specs and testing procedure for the ignition and perhaps buck that nub off and reset the megneto.

I'd really like this saw to run like the $500Cad out the door saw this is supposed to run like... or run as well as one would assume a $500 tool should run.


Get OEM for both the carb and gasket if at all possible. I just rebuilt a blower same issue, went the China way..you know, whole fuel system for 10$ shipped. It ran but was hard to start and would not rev all the way up. I just got back to OEM yesterday totally different deal.

Yeah I don't really get how China is bucking off respectable complete fuel kits/carbs/everything included for $9.99... seems shady. Kinda like taking a quality OEM spicer u-joint out of a driveshaft then putting in a guaranteed piece of $#!# moog. lol

I'm not replacing the carb... yet. I just meant the carb/intake gasket meaning the carb to intake gasket. I'm under the belief the carb is working the way it is designed to and the ports are clean. It also hasn't been molested by brushes, pik's, ramming the needles home, etc... gonna hold off on replacing the carb.
 
Tinman204

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For what its worth I just had an identical ms250 in my shop that sounds like it had the same type of running issues..

It ended up being a bad carb. I swapped out the crappy zama for a good walbro carb I had and the saw ran perfect after..
 
husqORbust

husqORbust

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For what its worth I just had an identical ms250 in my shop that sounds like it had the same type of running issues..

It ended up being a bad carb. I swapped out the crappy zama for a good walbro carb I had and the saw ran perfect after..

What made the carb bad? Bad as in brand new and FUBAR'd/improperly machined or as in time of day couldn't have been given to diag and repair the carb?
 
Tinman204

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High speed curcuit was plugged.

I put a new carb kit in it and it changed nothing.

The saw would start and idle but would die the minute I touched the throttle.

No idea what caused the carb to go bad as it wasn't my saw.

Zama carbs these days are finicky and unreliable IMHO.
 
Patrick62

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all these little carbs have incredibly tiny passages. I cant blame ya for replacing it, how in the heck would you clear it out? Some of the older stuff actually had a removable main nozzle...

The 250's that I have started and adjusted at the store (new) all would cut well, had surprising throttle response. Handled very nicely, and even had me thinking that one of those in the fleet would be fun.... (CAD is incurable).
 

SEAM

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...completely forgot about the accelerator pump/piston - the MS250 carb has one, I believe.
A worn piston will cause symptoms like unreliable, poor or no acceleration from idle rpm. The only reliable cure for this would be a new carb.
 
Tinman204

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all these little carbs have incredibly tiny passages. I cant blame ya for replacing it, how in the heck would you clear it out? Some of the older stuff actually had a removable main nozzle...

The 250's that I have started and adjusted at the store (new) all would cut well, had surprising throttle response. Handled very nicely, and even had me thinking that one of those in the fleet would be fun.... (CAD is incurable).

I couldn't get that carb unclogged. Even soaked it for a week in carb cleaner, didn't work.

The guy I repaired it for needed the saw so I built him the carb off a parts saw i have here.
 

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