MS290 won't idle

[RedOctobyr]

Hi all,

I bought a used MS290 from Craigslist. It will start nicely, and will run at full throttle, but dies as soon as you let it idle. The best way to keep it limping along is to keep pulling/releasing the throttle.

It shows 150-155 psi of compression. I looked at the piston from the intake side, it looks good to me. I haven't taken the muffler off yet.

I've read that cracked fuel lines are common on these saws, and can cause similar symptoms. I replaced the fuel line today with an OEM Stihl line, no change. The fuel filter looks basically perfect, so I didn't replace it. I re-set the carb needles per the markings on the housing, 1/4 turn for L, 3/4 turn for H, no change. I tried opening the L needle as far as the limiters allow, it still won't idle.

From looking at the service manual, removing the multiple covers to access the impulse line looks like kind of a pain Does a cracked impulse line seem consistent with my symptoms? I'll certainly take the covers off if the impulse line should be checked, but I'm just trying to target the next most-logical step. My local dealers don't seem to have the impulse line in-stock, so I can't swap it quickly anyhow.

I can order a carb rebuild kit, maybe the carb is just dirty. The dang limiter caps sound like they'll cause trouble, though, since I don't have the removal tool (apparently 5910 890 4500), or replacement caps.

I have an ultrasonic cleaner. If I can't remove the needles yet, is there anything to be gained by removing some of the carb's covers to open it up as best I can, and running it in the ultrasonic cleaner? Or is that useless if I can't first take the needles out?

Thank you.

 

[Poleman]

I would defiantly remove the limiters. You can do that with a deck screw and patience. I can't remember how the limiters are but that should do it. Your on the right path with richening the L screw. I could possibly be a dirty carb that needs a rebuild or the impulse line, but I would start with getting full adjustment to the H & L and go from there.

 

[Cope1024]

Change the fuel filter too. Looks can be deceiving.

 

[Wood Doctor]

Opening the low speed set screw lowers the idling by making a richer mixture. Did you raise the idle screw setting (clockwise) to maximum already? I cannot see where you mentioned that in post #1. The idle setscrew is separate from the lo and hi speed setscrews.

 

[RedOctobyr]

Poleman, maybe I'm thinking of another saw model I read about, but I thought the limiter caps also serve to add enough friction to keep your carb settings from drifting? That is, I thought you need to reinstall the limiter caps, to make sure the needle screws don't vibrate in or out. Is that still true here? If I simply need to remove the caps and leave them off, that's one thing. If I also need to replace them, that's a bit more hassle.

If I can't get it to idle properly within the limiters range, I'm assuming something is wrong, and needs to be addressed, otherwise I'm just masking the root of the problem? I'm basically at sea-level.

Cope1024, interesting. It doesn't have crud stuck to the mesh, or anything like that. But that's a fair point, I shouldn't just ignore the filter.

Wood Doctor, I did not actually try raising the LA idle-speed screw setting. I justified that by the fact that even with constantly getting on/off the throttle, at an RPM that should be above idle, it still won't really stay running. So I'm not really relying on the idle-speed adjustment screw. But I don't have a chain for it yet, so I've lost the RPM reference of whether the chain is turning or not (turning = above idle). I'll try speeding up the idle.

 

[Wood Doctor]

Poleman, maybe I'm thinking of another saw model I read about, but I thought the limiter caps also serve to add enough friction to keep your carb settings from drifting? That is, I thought you need to reinstall the limiter caps, to make sure the needle screws don't vibrate in or out. Is that still true here? If I simply need to remove the caps and leave them off, that's one thing. If I also need to replace them, that's a bit more hassle.

If I can't get it to idle properly within the limiters range, I'm assuming something is wrong, and needs to be addressed, otherwise I'm just masking the root of the problem? I'm basically at sea-level.

Cope1024, interesting. It doesn't have crud stuck to the mesh, or anything like that. But that's a fair point, I shouldn't just ignore the filter.

Wood Doctor, I did not actually try raising the LA idle-speed screw setting. I justified that by the fact that even with constantly getting on/off the throttle, at an RPM that should be above idle, it still won't really stay running. So I'm not really relying on the idle-speed adjustment screw. But I don't have a chain for it yet, so I've lost the RPM reference of whether the chain is turning or not (turning = above idle). I'll try speeding up the idle.

I usually start the whole tune-up process by raising the idle slightly because I know I can always cut that back as needed to slow the saw down. The idea is to get a smooth acceleration to WOT and then drop down the idle with the idle set screw so that the chain is barely creeping along or stops but the saw continues to run.

I remove the limiter caps only as a last resort. This can be done rather easily with a #10 wood screw that grabs the plastic and allows you to pull them out.

 

[DSS]

A saw with an air leak usually won't idle. Not at a normal speed, anyway

 

[Poleman]

That's usually true but its usually an increased idle speed since saw is getting more air and leaning mix. My understanding here its a slow idle then quits. I guess I did overlook just increasing idle speed as stated earlier. I was thinking after wot then letting off it dies indicates lean condition

If the H & L needles don't have springs I put a piece of fuel line in to put pressure on them to hold them stationary.

 

[Cope1024]

Poleman, maybe I'm thinking of another saw model I read about, but I thought the limiter caps also serve to add enough friction to keep your carb settings from drifting? That is, I thought you need to reinstall the limiter caps, to make sure the needle screws don't vibrate in or out. Is that still true here? If I simply need to remove the caps and leave them off, that's one thing. If I also need to replace them, that's a bit more hassle.

If I can't get it to idle properly within the limiters range, I'm assuming something is wrong, and needs to be addressed, otherwise I'm just masking the root of the problem? I'm basically at sea-level.

Cope1024, interesting. It doesn't have crud stuck to the mesh, or anything like that. But that's a fair point, I shouldn't just ignore the filter.

Wood Doctor, I did not actually try raising the LA idle-speed screw setting. I justified that by the fact that even with constantly getting on/off the throttle, at an RPM that should be above idle, it still won't really stay running. So I'm not really relying on the idle-speed adjustment screw. But I don't have a chain for it yet, so I've lost the RPM reference of whether the chain is turning or not (turning = above idle). I'll try speeding up the idle.

RedOctobyr, When I did the muffler mod on my MS290 my dealer pulled the limiter caps for me. Unless you cut the limiter tab off, you're back to square one. I recommended changing the filter because of my experience with one that looked OK but was clogged. Besides, it's cheap insurance.

 

[RedOctobyr]

Thank you guys, great info.

As far as the idle condition, sorry if I described it poorly. It's not a slow idle that dies. The engine basically will only run "properly" at full-throttle. If you let it slow down, it will promptly die, unless you keep "pumping" the throttle to try and keep it going. If you have it at mid-RPM and give full throttle again it seems to kind of hesitate briefly, then it speeds up.

Because it isn't working properly, I haven't tested simply running it at full-throttle for a minute or similar, to see if it will keep running indefinitely. I'm afraid of damaging the piston/cylinder if I try a test like that.

On the filter subject, I just dumped out the tank and put in fresh fuel (so there should be fewer bits of junk floating in the tank). I wonder about simply pulling the filter off, and starting it once, briefly. If it's suddenly fine, then the filter is the problem. If there's no difference, then the filter is less likely to be the problem (replacing it is still probably a good idea, but it might be less urgent). Then reinstall/replace the filter, as appropriate. This risks sucking something in the carb, of course, but I may have to clean the carb anyhow. I probably can't get to a dealer during business hours for a few days anyhow, so I can't just replace the filter today even if I wanted to.

Unless I suddenly solve this, I will try removing the limiter caps, if for no other reason than to let me remove the needles for cleaning the carb.

I hope it doesn't have an air leak from around the crankshaft, or some other issue that needs special tools, etc. I don't have the tools to do a crankcase pressure/vacuum test. But one step at a time.

 

[Cope1024]

Thank you guys, great info.



On the filter subject, I just dumped out the tank and put in fresh fuel (so there should be fewer bits of junk floating in the tank). I wonder about simply pulling the filter off, and starting it once, briefly. If it's suddenly fine, then the filter is the problem. If there's no difference, then the filter is less likely to be the problem (replacing it is still probably a good idea, but it might be less urgent). Then reinstall/replace the filter, as appropriate. This risks sucking something in the carb, of course, but I may have to clean the carb anyhow. I probably can't get to a dealer during business hours for a few days anyhow, so I can't just replace the filter today even if I wanted to.

Unless I suddenly solve this, I will try removing the limiter caps, if for no other reason than to let me remove the needles for cleaning the carb.

It's your saw, but I would not run the saw without a fuel filter. I would remove the limiter caps. You will want to do a muffler mod one day.

 

[RedOctobyr]

I checked the tank again, it was clean. So I made a judgement call and tried it with no filter. No improvement. Put the filter back on.

The #10 wood screw trick worked great for getting the limiter caps off, thank you! The caps are off. I set the needles to 1 turn out, per the manual, no change. I opened them another 1/2 turn, it still didn't seem any better. I could have kept trying, but if it still isn't getting enough fuel with them opened a fair amount, I took that as a hint that something is wrong.

I took the carb off again, I'm going to run it through the ultrasonic cleaner, with the needles removed, maybe that will help.

I did try putting a small zip-tie around the impulse line where it connects to the carb, in case it was sucking in air at the hose connection (the hole in the impulse hose is larger than the hole in the new fuel line, and the nipples they connect to are the same size), but that didn't seem to do anything.

 

[RedOctobyr]

Success!!

I disassembled the carb as much as I dared. Took off the top and bottom covers and diaphragms, removed the "Fixed Jet", and took out the High and Low needles. The inlet screen looked clean. I didn't realize that these carbs have a last-resort filter in them; that's good. There was some brown crusty gunk under the pump diaphragm; the rest looked clean to me.

I ran the disassembled carb for an hour in the ultrasonic cleaner, using Simple Green HD as a solution (it's safe for aluminum), at 140F/60C. Afterwards I rinsed it with water, then blew it off with air, then sprayed it with carb cleaner. The carb cleaner was in part to help get rid of any residual water trapped inside. It came out of the ultrasonic looking great, the obvious brown gunk was gone.

I reassembled it, trimmed the ears off the limiter caps, put them on, and set both the H and L needles to 1 1/2 turns out (manual says 1 turn, but I wanted to play it safe).

It fired up, and finally 4-stroked nicely (and smoked) at full-throttle. Let off the trigger, and it idled Awesome!! Once I get a chain on it, and/or pull a tach off one of my other machines, I'll set the idle speed, then try and tune it while actually cutting.

Thank you all for your help, I really appreciate it

 

[Cope1024]

Congratulations, you're now a chainsaw mechanic. Glad you got it sorted out.

 

[Wood Doctor]

"There was some brown crusty gunk under the pump diaphragm; the rest looked clean to me. "
-------------------------
That was probably 90% of your trouble. BTDT. This gunk stuff will also rob the engine of its power at WOT. I have fixed several carbs this way without going any further, except perhaps a 2-day soak in mixed fuel and a cleaned up filter screen.

Your ears are also a pretty good tach. Don't sell them short.

 

[RedOctobyr]

Hmm. I finally got the sprocket I needed, and put a chain on the saw. I haven't run it for a few weeks.

Tonight it started nicely, and again sounded good, 4-stroking (and smoking quite a bit) at WOT, for a few seconds. Then, still at WOT, it suddenly sped up and the smoke decreased. It did this several times. I still have it set quite rich, about 1 1/2 turns out L and H. It also won't stay idling (but my idle speed may simply be set too low).

So something is going on. Either it's also changed since I got it running a few weeks ago, or I simply didn't notice before.

When I got the saw originally, it wouldn't idle. I replaced the fuel line (which didn't seem to help), then I cleaned the carb, and it suddenly ran well. I have not yet checked/replaced the impulse line; I'm not sure how difficult it is to access.

 

[huskihl]

That sounds like an air leak. My jred was sucking air through the throttle shaft openings in the carb and did the same thing

 

[RedOctobyr]

Ugh Chasing down an air leak, and not having the proper tools (vacuum pump, etc) doesn't sound like fun.

How did you find, and fix, your throttle shaft leak problem? Can pulling that shaft and maybe adding some grease to it help seal up a leak?

Sent using Tapatalk.

 

[huskihl]

I squirted carb cleaner around everything and found it.

Ugh Chasing down an air leak, and not having the proper tools (vacuum pump, etc) doesn't sound like fun.
I think what it's doing is: it's such a small leak that only appears to leak about 3 seconds after full throttle. Mine would 4 stroke then 2 seconds later lean out and then race. I sprayed white grease around the whole carb and let it set for a couple days. Problem gone.
My carb is nla. I took it off and pulled the brass plate out and removed the shaft. Took an old but still soft metering diaphragm and cut a "gasket" out of it around one of the screw holes, which happened to be a very tight fit around the shaft. I don't recommend it as a permanent fix, but I had nothing to lose, as it was easy to tell if it still leaked due to the saw racing 2 seconds after full throttle. Fixed it.
Yours could be a loose or cracked intake boot, or a crank seal also
How did you find, and fix, your throttle shaft leak problem? Can pulling that shaft and maybe adding some grease to it help seal up a leak?

Sent using Tapatalk.

 

[huskihl]

I don't know why it quoted in your quote, but read it. My fix is in there^