Stihl MS880 runs at high idle then dies.

[hotshot]

I'd disconnect the throttle linkage, pull the carb one more time, and make sure the throttle shaft, spring, & plate are correctly assembled. The little air bypass hole in the round plate should be at the bottom & the throttle plate not bound up when you fully adjust the LA screw counterclockwise...

Leave the linkage unhooked when you reinstall the carb, start it up and see if you can now adjust the idle properly with the LA screw.

If it still does idle high, there's a vacuum leak somewhere & if you run the saw for long it can quickly roach the piston & cylinder. What vacuum did you test to? Should hold -7"HG to -10"HG for an hour if the seals are good.

 

[splitpost]

also check if the trigger is comming back all the way and that no hoses are fouling around the carb and linkages

 

[rxe]

Throttle plate in back to front? It has a bevel...if back to front it doesn't seal properly.

If there are no leaks, the the only way for air to get in (and cause the high idle) is either because:

- the throttle is not closing: rod bent, as above
- plate in back to front
- plate bent

 

[ronT2]

Is #5 shown in the IPL missing? I don’t see it in your parts pile. If it’s missing you might not be moving the needle off the seat.

I’d also stick with a single pump diaphragm as others have said. I wouldn’t stack two of them, one gasket and one diaphragm as shown in the IPL.

I just swapped out a rubber pump diaphragm for a clear blue plastic like diaphragm on a saw that was running or….not running like yours. That solve the whole problem, saw runs great again.

 

[chad556]

Hey guys thanks for all the advice, I am going to go over the carb with our mechanic tomorrow to see what is up with it. If everything looks ok and a little adjustment wont give any promising results then I am going to recheck for an air leak.

The vaccum test that I did only held vac for about 10 minutes at 10"hg. This didnt seem good but my subsequent pressure test showed only the innertube seal that i had made to seal up my exhaust port was leaking air. Since i had so much trouble sealing up the muffler I was quick to just call the test good and move on.

The muffler doesn't bolt on like most everything else i have seen. Its clamped on by a thick wire and two little wedge pieces. Therefore its almost impossible to get a firm seal around it with a piece of rubber. I am thinking of maybe using a liquid gasket material to try and seal it. If anyone has any experience dealing with this and could let me know what to do i would greatly appreciate it.

I just swapped out a rubber pump diaphragm for a clear blue plastic like diaphragm on a saw that was running or….not running like yours. That solve the whole problem, saw runs great again.

Oh and I do have part #5 in there, the little screw that holds that tiny rod in place. I have also swithed to the black rubber pump diaphragm, but as before, no luck when using the gold, black or black and gold together for that matter. I don't think thats the problem but thanks for the suggestion. Do you know were I could find a diaphragm like that blue plastic one for this carburetor.

 

[ronT2]

Oh and I do have part #5 in there, the little screw that holds that tiny rod in place. I have also swithed to the black rubber pump diaphragm, but as before, no luck when using the gold, black or black and gold together for that matter. I don't think thats the problem but thanks for the suggestion. Do you know were I could find a diaphragm like that blue plastic one for this carburetor.

Oops…sorry I should have noticed that. The blue plastic diaphragm came in a rebuild kit for a much smaller carb. I don’t know if they’re available for a carb on an 880.

And one more thing, sorry to go on and on but it might be worth mentioning: When i took the fuel hose out of the carb after getting the saw to run(unsuccessfully) it spewed gas out in a 12" high stream that slowly died down over about 10 seconds, leaving my gas tank almost empty and me partially covered in mixed gas. Is there supposed to be pressure build up in the fuel tank like that? The mechanic I know didnt think it was right but wasn't sure. He said the tank should be vented and shouldnt shoot gas like that. I think hes right but I'm not sure.

Could this be causing the symptoms? Is it even a problem? I'm pretty stumped here there are about 4 things that could possibly be wrong with this thing and I am not sure where to start.

That doesn’t sound right. From what you’re describing it sounds like the pump diaphragm isn’t working correctly. It almost seems like the impulse is somehow pressurizing the fuel tank. Here’s a link to some more info that might be helpful.

Walbro WG-8 Carb Disassembly

 

[rxe]

Fuel tanks do pressurise, but this should not be causing a high idle. If anything, it should be flooding the saw out. Check the tank breather, it might be blocked. Some modern units do deliberately pressurise the tank (MS192, KM130 that I know of), I haven't torn into an 880 to see if it is the same.

If you have a high idle, air is getting in somewhere - sounds like through the carb, as you've pressure tested the saw.

 

[willbarryrec]

The vaccum test that I did only held vac for about 10 minutes at 10"hg. This didnt seem good but my subsequent pressure test showed only the innertube seal that i had made to seal up my exhaust port was leaking air. Since i had so much trouble sealing up the muffler I was quick to just call the test good and move on.

The muffler doesn't bolt on like most everything else i have seen. Its clamped on by a thick wire and two little wedge pieces. Therefore its almost impossible to get a firm seal around it with a piece of rubber. I am thinking of maybe using a liquid gasket material to try and seal it. If anyone has any experience dealing with this and could let me know what to do i would greatly appreciate it.
.

One thing I have had good luck with for vac tests is cleaning the outside of the exhaust port with some carb cleaner and using some Gorilla brand duct tape to seal the exhaust port....only works for vac testing but so far it's worked for me.
Hopefully I won't learn I'm a idiot here in a post or two. :msp_rolleyes:

Good luck!
Keep on working on it and don't give up it will be well worth it,learning how to work on saws not to mention your own 120cc saw!

 

[JJay03]

One thing I have had good luck with for vac tests is cleaning the outside of the exhaust port with some carb cleaner and using some Gorilla brand duct tape to seal the exhaust port....only works for vac testing but so far it's worked for me.
Hopefully I won't learn I'm a idiot here in a post or two. :msp_rolleyes:

Good luck!
Keep on working on it and don't give up it will be well worth it,learning how to work on saws not to mention your own 120cc saw!

Think this would work for pressure test also if you then bolted on the muffler after using the tape? Same for covering the intake then bolting on the boot?

 

[RiverRat2]

Good luck!
Keep on working on it and don't give up it will be well worth it,learning how to work on saws not to mention your own 120cc saw!

I think he ought to box it up and send it to me,,,,, and let me straighten it out for him!!!!!!!:msp_mellow::msp_ohmy::hmm3grin2orange:

 

[chad556]

Hey guys, thanks for the encouragement and advice. Our mechanic has been sick so he hasnt looked at the saw yet but I have the engine blocked off and tried out a vac test again. i sealed up the muffler with a piece of innertube and some form-a-gasket liquid stuff that i let cure overnight. It seems like it is failing the test it will only hold 10"hg for a couple seconds, quickly droping down to 5-7"hg and then slowly falling to zero after about 2-5 minutes. Maybe it is just a leak in my muffler seal again but I will try to be more precise with my pressure test this time and actually use a pump with a pressure gauge on it instead of trying to 'eyeball it'. It has been one heck of a learning experience, thats for sure.

In preparation for the pressure test I have ordered a flywheel puller (i can always sell it again once im done with it) and an entire kit of gaskets and seals for the crank case. Goodbye $80 but I figure I am already over $800 into this saw so what is a couple more bucks. Hopefully I can make a little money with it once I fix it up and from what I have seen lately on ebay $880 for an 880 isn't bad lol.

 

[chad556]

Ok, I have an update on the situation. The other day I finally was able to screw a pipe into the flywheel and weld a nut into it and, after several hours of frusteration, finally get the thing to pop off.

Underneath i found out that the seal was cockeyed a bit, flush on one side, sticking out by a mm or so on the other. This would be the cause of the symptoms im experiencing right? Now my question is: Should i try to just tap the seal back into place or should I try to replace it with a new one(i have the seal and gasket kit)? I would hate to put this together and have it not work.

I also sealed off the muffler with a rubber tube and gasket maker so i will test the pressure holding before I go ahead with reassembly. I think I have found the answer to the problem though. Any insight from successful air leak fixers would be great :smile2:

 

[betterbuilt]

You could put pressure to it. Then put some oil over the seal and see if it bubbles.

 

[gink595]

Air leaks suck! I'd put a new seal in it, or tap it in and see if it leaks. That's a judgement call there.

 

[hotshot]

Gink is exactly right....if any doubt, pull it out & replace it.

I found that you can slip an oiled toothpick between the shaft & an old questionable seal that has lost most of it's memory (like I have), & then the seal will refuse to set back down onto the shaft for up to three or four minutes. Vacuum testing will drive you nuts on those old intermittent seals.

 

[chad556]

Thanks for the continued support. I decided to replace the seal, tapped the new one in and redid the pressure/vac test. This time I used Ultra Blue Gasket maker around my muffler port and stuck my rubber piece there overnight. It is air tight.
Unfortunately the rest of the saw wasn't. I checked and checked and couldnt find how this thing was losing air. The pressure test is hard because I am using a bike pump that slowly looses pressure on its own. The vac test is solid though and does not lie to me at least I think (I am suspicious that the decomp valve doesn't have enough strength to keep air from slowly seeping in).

I was about ready to give up when I remembered something I read here about dunking the saw. Sounded like just the crazy Idea that i needed to try, Heres how it went:

[video=youtube;CdXG8nH20Mk]http://www.youtube.com/watch?v=CdXG8nH20Mk[/video]

Is that leak from the crank case that bad? It is at the bottom of a tiny little "hole/depression" molded right into the metal of the crank case itsself. It seems like just a tiny crack or pinhole, couldn't even see it with a magnifying glass. To be safe I put some epoxy around it and blew it around with compressed air until it was coated all around the area. I then hooked up my vac pump in hopes the vacuum would draw in the epoxy and make the patch job better. It held vacuum 10"hg for about 20 minutes. I then started to put the saw back together and then pumped up the vac pump agian and left to go home. If it is still holding tomorrow it will be very encouraging. I will dunk it one more time tomorrow to see if the leak is really fixed or not and if so I will put it back together and cross my fingers again.

I really wish I had tested that crooked seal now. It would have been nice to know if that was 100% surely 'the' leak or not. Oh well. If the vac/pressure tests are passed and I have two carbs to play with i can't be too far away from solving this mystery. I just want to cut some wood with this baby. :chainsawguy:

 

[chad556]

Well, the good news is that it held a vacuum for 20 hours overnight. I left it at 10"hg and when i came back it was still holding 7"hg. I think thats safe to say there there is no significant air leak there right?

So I excitedly put the saw back together for what I hoped was the last time. Fired her up and low and behold it was the same problem all over agiain. Two different carburetors both had the same problem as well and I have tried at least 3 different batches of fuel. Unless anyone has any other ideas I should try I think its getting to the point where I have to decide weather to sell it whole as not running or cut it up and sell it piece by piece:msp_sad::bang: Its getting harder and harder to pull that cord knowing that its probably going to be the same old bad news again and again.:help:

 

[GA_Boy]

what about the impulse line? Have you looked at the intake boot?

 

[Andyshine77]

This may sound stupid, but have you been using the same spark plug? I've had a few plugs go bad, and they can really make one scratch their head.

 

[betterbuilt]

Don't give up yet. I think you need another set of eye's on it. I have a 066 that's doing something similar. I just have to take a brake from it and go back to it when I have the patience to look at it again.

I bought all new rubber lines to replace the ones I had just bought because the air leak wouldn't go away. Have you tried a pop off tester like Gink recommends in the thread I started.
http://www.arboristsite.com/chainsaw/171064.htm

I made one from the parts I had bought for pressure testing my saws.