STIHL MESSING WITH MY SAW ms250

SteveSr

SteveSr

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These carbs can develop leaking / intermittent main nozzle check valves. Saw runs right one day but not the next. If it sticks closed the saw will idle fine but won't rev up. If it leaks open it will run at WOT but might die at idle. These are part of the main nozzle assembly and are not repairable.

Search this board for more info and how to test.
 
husqORbust

husqORbust

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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.

Checked the high speed "check valve" based on Walbro's instructions: Blow into the main jet should flow free, suck and shouldn't flow free.

Mine seems pretty "unplugged", except perhaps the check valve is a smidge sticky??? Takes a smidge of lung pressure to push it off it's seat and let air flow, then almost any vacuum would suck it back into the closed position. If it does infact need a smidge of pressure to pop it off it's seat/open, then I wouldn't say it is sticky.

In any event, based on my few minutes of reading a non operational or sticky check valve does seem like it would cause my problems.

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).

Are you saying it's actually a throw away saw at US dollars but we're getting hosed up in Canada? o_O

...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.

It's a C1Q-S242A (or right underneath says 424a)... This carb has an accelerator pump? Junk... how can I better diag this?

If so, based on my few minutes of reading if it were fubar'd it would also cause the same issues my saw is having. What a terrible implementation of design.

Wt-215 it's a three screw carb. 30. Bucks.

Just by default I should be buying one of these and slapping it in the hole to overcome what sounds like guaranteed failure via the accelerator pump in Zama's...

The WT-215 is direct drop in, linkages, impulse port, and all?
 
Patrick62

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The only saw I ever worked on that had a accelerator pump was a mid range Husky. A spring loaded ball against a throttle shaft, the carb went wonky on us, I sourced a replacement as I could not get that carb to work correctly.

Yea, kind of a cheaper made clamshell engine in the things. But the things will hold up quite well!
 
husqORbust

husqORbust

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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.

The only saw I ever worked on that had a accelerator pump was a mid range Husky. A spring loaded ball against a throttle shaft, the carb went wonky on us, I sourced a replacement as I could not get that carb to work correctly.

Yea, kind of a cheaper made clamshell engine in the things. But the things will hold up quite well!

https://www.arboristsite.com/community/threads/200t-carburetor-accel-pump-block-off.238372/

My carb doesn't have that brass plug, but is does have the machined hole to be able to see the throttle shaft go through the carb, and it does have a machined flat on it.

Possible this mods already been done or my carb doesn't have the accelerator pump to begin with?
 
Tinman204

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The only saw I ever worked on that had a accelerator pump was a mid range Husky. A spring loaded ball against a throttle shaft, the carb went wonky on us, I sourced a replacement as I could not get that carb to work correctly.

Yea, kind of a cheaper made clamshell engine in the things. But the things will hold up quite well!

The ms200t is notorious for accelerator pump failures.

OP, if you think the carb is no good then just replace it, carbs are cheap for the ms250, not really worth the time trying to repair it.
 
husqORbust

husqORbust

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OP, if you think the carb is no good then just replace it, carbs are cheap for the ms250, not really worth the time trying to repair it.

Ya... kind of what I'm gathering. Either the carb is good and just needs a diaphragm/needle or it's junk. Either way its $65 cad to my door for a wt215. If all my carb ever needed was a diaphragm I cant see replacing it.

As far as I understand my carb doesnt have a accelerator pump. If it needs a check valve my carb is getting thrown at an ultra high rate of speed into the nearest garbage receptacle and a wt215 will be on the way 6 minutes later.
 
farmer steve

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Ya... kind of what I'm gathering. Either the carb is good and just needs a diaphragm/needle or it's junk. Either way its $65 cad to my door for a wt215. If all my carb ever needed was a diaphragm I cant see replacing it.

As far as I understand my carb doesnt have a accelerator pump. If it needs a check valve my carb is getting thrown at an ultra high rate of speed into the nearest garbage receptacle and a wt215 will be on the way 6 minutes later.
Like Tinman said 250 carbs are cheap. I don't think mine was over $40 from the stihl dealer.
 
Chris-PA

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As I recall the carb screw grommet will need to be changed. I believe that the WT-215 was an option on the early 250s so the correct grommet should be listed in the parts list.
Exactly. Another ZAMA with a leaky check valve, no surprise. Put a Walbro on it.
 
husqORbust

husqORbust

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Put the new gasket and carb spacer on. Reset the needles to 1 out on the low and 1.5 out on the high (as per Stihl's zama spec)


Saw was sitting inside for a few days so was indoor room temp. Pretty sure the crankcase had excess fuel in it from when I was running it before at the 8/2.5 needle settings which is likely why it fired up on full choke.

First pull was full choke and it fired, I clicked it to high idle and didn't touch the throttle. Died on its own after sputtering around.
Second pull was on high idle. Died as soon as I touched the throttle.
Third pull was with the switch just in the "on" position. I didn't actually pull the throttle when it was running, force of habit my hand went straight to the trigger.
Last three pulls were just "on"

Started messing with the carb:
1/4 then 1/2 turn leaner on the low didn't change anything. Up to 1 turn richer on low didn't change anything. I went ahead and turned the low out 8 turns and it fired up and idled and bogged through a rev up and eventually was full revving.

After scratching my head for a minute I went back outside and figured I'd try to simulate if it were the check valve...too dark for a video... but guess I could have done it straight audio lol. so will have to do with my explanations:

Turned the low screw back in to about 1 turn.
pulled, idled as long as it did on my last three pulls in the video. Squeezed throttle wide open, died.
Did that again, died.
Did that again, died.
Did that again, felt like it wanted to rev up and died.
Did that again, feathered and worked the throttle upto max rpm then died as soon as it was idling for a second.
Fired it back up, was able to feather it off idle to WOT, back down to an idle for less then a second and pinned it and it bogged down then opened up to max rev.

Definitely seemed like it was acting like the check valve. My theory is richening up the Low the 8 turns masked the air getting into the check valve, and then assisting the extra air when the throttle blade opens to have less of a lean bog transition to max rev.

New carb is on the way.
 
husqORbust

husqORbust

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For S&G's while I await my new carb I pulled the carb off and took the throttle rod out. Out popped an accelerator pump/piston.

Bore and piston look mint. o ring probably doesnt seal worth a hoot anyways. Guaranteed to fail. Silly design.
 
husqORbust

husqORbust

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Carb came in today. Pretty much direct drop in like was said. I had to trim off some of the plastic casting around where throttle lever wanted to sit. Not a big deal, just used a razor blade and shaved it until it fit. Then I had to take off about 20 file strokes the part of throttle rod that hooks through the lever. Was having slight interference achieving full throttle without doing that. Guess the throttle lever is just a tighter fit to the carb body than the Zama is.

New carb seems to have fixed all my issues. Here's a video... it's a bit long. I've listed some times off if you want to skip around through the video for the meat and potatoes otherwise it's an unedited video from pull 1 to where I figured was a decent enough setting. Think she's mostly running good now with respectable needle settings... 1.5 out from seated on both needles was pig rich.

I reckon I ended up 1 turn out on the high and right around 3/4ish on the low. Idle might be a tickle too high still. Will have to bury it in some wood and fine tune it a bit more but hopefully she keeps firing up and can hold an idle and rev up on demand.

12:20 on is the meat and potatoes if you don't want to see someone try to mess around with a pig rich saw.

Because there's no commentary here's some explanation of what I was doing....

Right around 30 sec it's idling in the high idle position with no high idle. So I pinned the idle screw, still no high idle. Tried between high and low idle for a couple minutes seeing if I could get it to do anything trigger wise.

3 minute mark I start leaning out the Low. Ended up messing with the low for awhile to see how it was reacting.
5:50 mark Start dialing in on a closer low setting. Still pig rich. Won't rev up but doesn't bog out on throttle.
8:45 start making real progress on the low needle.
11:20 start thinking about taking the idle screw off full pin.
12:20 I try to find my rich and lean range for the low. Put it right around the middle of the two points.
13:30 I hold it wide open and richen the high to see if the part throttle bog is rich or lean induced. It was rich induced, start leaning it out.
16:00 I decide the low is right in the ballpark. Snappy throttle response. Start dialing the idle down again. Tickle rich maybe?
17:30 roll it around a bit. Good to go.
18:00 check function of high idle start.
18:45 is idle to pinned for a second. Likely still a smidge rich on the top end.


Next I'll go bury it in some wood and see what happens.
 
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