Stihl MS880 runs at high idle then dies.

[chad556]

Both Impulse line and intake boots are fine, passed visual inspection and didnt leak for pressure or vac test. I have used two different spark plugs. One that was brand new and worked just fine in an MS441, good advice though, its always the spark plug (unless you have my luck, lol).

I am intrigued by your thread, betterbuilt. A leak in the carburetor would explain a lot. The original carb I rebuilt (several times) with no luck, used an 084 carb kit because thats all I could find. I also picked up a used 084 carb on ebay for 40 bucks and that had the exact same problem as the original 880 carb. Could they both have leaks? And how do you test them, I am pretty sure i could round up the parts to make a tester but what do I hook up to what? and what do I plug?... and what am I hoping to see for that matter? I'm wrapping my mind around this stuff somewhat better but carbs are still like magic to me so small yet so much delicate stuff going on in there.

 

[RiverRat2]

I am intrigued by your thread, betterbuilt. A leak in the carburetor would explain a lot. The original carb I rebuilt (several times) with no luck, used an 084 carb kit because thats all I could find. I also picked up a used 084 carb on ebay for 40 bucks and that had the exact same problem as the original 880 carb. Could they both have leaks? And how do you test them, I am pretty sure i could round up the parts to make a tester but what do I hook up to what? and what do I plug?... and what am I hoping to see for that matter? I'm wrapping my mind around this stuff somewhat better but carbs are still like magic to me so small yet so much delicate stuff going on in there.

a squeeze bulb like a blood pressure tester cause it has a check valve /get a small pressure guage, 0-30 psi, and a T fitting to put the guage inline with the hose and an assortment of hose barb fittings to slip in the pressure hose,,,

with the carb and fuel system intact, remove the fuel tank cap and use a wire hook to fish the pick up body (fuel filter) and fuel line end out of the fill hole, remove filter,, insert the pressure tester hose barb fitting in the end of the fuel line where the pick up body was and pressure up to approx 17 psi it should hold that pressure indefinitley

 

[betterbuilt]

Both Impulse line and intake boots are fine, passed visual inspection and didnt leak for pressure or vac test. I have used two different spark plugs. One that was brand new and worked just fine in an MS441, good advice though, its always the spark plug (unless you have my luck, lol).

I am intrigued by your thread, betterbuilt. A leak in the carburetor would explain a lot. The original carb I rebuilt (several times) with no luck, used an 084 carb kit because thats all I could find. I also picked up a used 084 carb on ebay for 40 bucks and that had the exact same problem as the original 880 carb. Could they both have leaks? And how do you test them, I am pretty sure i could round up the parts to make a tester but what do I hook up to what? and what do I plug?... and what am I hoping to see for that matter? I'm wrapping my mind around this stuff somewhat better but carbs are still like magic to me so small yet so much delicate stuff going on in there.

You know we are having almost the same luck. I too bought a used carb of of ebay.

The tester was easy to build. It really tells you quickly you have problems. My carb was leaking air every time I turned my head. It seem strange but apparently my skills for rebuilding a carb weren't up to par. I never did find anything in my carb but it definitely wouldn't seal about half the time.

The new used carb I got I didn't do anything to but open it up to be sure it wasn't all gross or missing parts. I put it on the saw. and it seems to run now. There is a problem with the low side but when I got the saw the Low side was turned out quite a bit so I bet there's more to it. The saw was running okay for the first try. Now I'll deal with that low side screw and see if I can find anything.

 

[betterbuilt]

a squeeze bulb like a blood pressure tester cause it has a check valve /get a small pressure guage, 0-30 psi, and a T fitting to put the guage inline with the hose and an assortment of hose barb fittings to slip in the pressure hose,,,

with the carb and fuel system intact, remove the fuel tank cap and use a wire hook to fish the pick up body (fuel filter) and fuel line end out of the fill hole, remove filter,, insert the pressure tester hose barb fitting in the end of the fuel line where the pick up body was and pressure up to approx 17 psi it should hold that pressure indefinitley

17psi. I've been using 10psi. Should I go higher?

 

[Saw Dr.]

Both Impulse line and intake boots are fine, passed visual inspection and didnt leak for pressure or vac test. I have used two different spark plugs. One that was brand new and worked just fine in an MS441, good advice though, its always the spark plug (unless you have my luck, lol).

I am intrigued by your thread, betterbuilt. A leak in the carburetor would explain a lot. The original carb I rebuilt (several times) with no luck, used an 084 carb kit because thats all I could find. I also picked up a used 084 carb on ebay for 40 bucks and that had the exact same problem as the original 880 carb. Could they both have leaks? And how do you test them, I am pretty sure i could round up the parts to make a tester but what do I hook up to what? and what do I plug?... and what am I hoping to see for that matter? I'm wrapping my mind around this stuff somewhat better but carbs are still like magic to me so small yet so much delicate stuff going on in there.

OK, I have not been following this until today. No offense, but it seems you are going about this the hard way... The rubber bits on saws are generally the weak spots. Even if the intake/impulse line passes a visual inspection, and pressure test, they could still be bad. Both the intake and impulse are part of the A/V system. The engine is not vibrating when you test it. Vibrations could be opening up cracks in the line that you don't see otherwise. If you have not changed the fuel line, that is a very inexpensive thing to try. I doubt two carbs are bad, but if you blew them out with high pressure air, they are bad now. There are check valves in those carbs that will be damaged by high pressure air. You should only use the little tube from a carb-cleaner can to clean them, or an ultrasonic cleaner.
I am interested in how you are testing the press/vac on the saw. If you are getting a good test, try flexing the a/v system while at it and see if the press drops. Also, where are you accessing the press to do the test? If you are using the impulse port, you are eliminating the impulse line from the test.
There are only so many places it could be leaking. The saw is obviously running very lean. You may also want to post up pics of the the mixture adjustment needle tips. I have seen a handful of carbs where "experts" have wound the needles in too tight, and broken the tips off when unscrewing them. Since the caps were missing when you got it, this is always an option.

 

[chad556]

OK, I have not been following this until today. No offense, but it seems you are going about this the hard way... The rubber bits on saws are generally the weak spots. Even if the intake/impulse line passes a visual inspection, and pressure test, they could still be bad. Both the intake and impulse are part of the A/V system. The engine is not vibrating when you test it. Vibrations could be opening up cracks in the line that you don't see otherwise. If you have not changed the fuel line, that is a very inexpensive thing to try. I doubt two carbs are bad, but if you blew them out with high pressure air, they are bad now. There are check valves in those carbs that will be damaged by high pressure air. You should only use the little tube from a carb-cleaner can to clean them, or an ultrasonic cleaner.
I am interested in how you are testing the press/vac on the saw. If you are getting a good test, try flexing the a/v system while at it and see if the press drops. Also, where are you accessing the press to do the test? If you are using the impulse port, you are eliminating the impulse line from the test.
There are only so many places it could be leaking. The saw is obviously running very lean. You may also want to post up pics of the the mixture adjustment needle tips. I have seen a handful of carbs where "experts" have wound the needles in too tight, and broken the tips off when unscrewing them. Since the caps were missing when you got it, this is always an option.

Wow, I have just been educated.

I did clean out my carbs with an air hose with the regulator turned down. They didn't work before cleaning though either but I was aggressive with the air and tried to get everything out of it that i could. It looks like i will need to test them both the be sure. The needles on the original one were fine though I took them out and they were both intact. These check valves in the carbs, can they be repaired or rebuilt?

My pressure testing procedure consists of blocking off the exhaust and the intake port between the rubber intake boot and the actual carburetor. I connect my pressure/vac hose directly to the impulse line so both the boot and impulse line are pressurized/vacuumed during the tests. I'm am using a Harbor Freight brake bleeder for the vac tests and a manual bike pump with a digital gauge for the pressure tests. Good point about the anti vibration system. I rotated the crank by hand during the test without losing anything but I should have checked the lines. especially when the thing was underwater.

I guess I will start with the carburetors and if they pass I will try to pinpoint something in the impulse line.

Thanks everyone for taking the time to share your knowledge, It means a lot and I am learning a ton:msp_smile:.

 

[chad556]

I almost got it I think...

[video=youtube;MU2fFRpsof4]http://www.youtube.com/watch?v=MU2fFRpsof4[/video]

So I finally had time to go through both carburetors again. I used the old 084 carb i got off ebay and took all of the best parts and built it up good and sprayed it down with carb cleaner. Installed it and got my video camera running to capture the first test.

I hate to ask a noob question after coming this far. But at this point, what the heck am I supposed to do now?

I have the carb set to factory settings, one turn out from the stop on both H and L screws and I set the idle screw as high as it would go(yes, i know how the idle screw works, one turn goes through the whole range) to start out with. I got the results you see in the video. I am afraid to touch this thing anymore because I am paranoid I will blow it up. When I rev it up it sounds like its revving a bit high to me and obviously, it idles low now, but too low to sustain its self. Compression with the new piston is 150+ psi, spark plug is brand new and spark is strong, gas is pretty new etc...

Does anyone notice anything I am missing here? In the past I have just fiddled with the carb to get it where I needed it, I don't really know what I am doing when it comes to adjustments and I will be the first to admit it. Can anyone explain exactly where I would go from here? L screw first? or H screw? Basically if it were you in that video where would you put the screwdriver and which way would you turn it? Thanks in advance guys, sorry for the noob question but i am terrified i will destroy this expensive beast with my inexperience. I am just about ready to take it to the dealer and have them tune it for me but I would rather learn and keep my money if some one is willing to teach!

 

[splitpost]

bring your idle up and then get your low side set

 

[StihlyinEly]

bring your idle up and then get your low side set

+1

I just saw this thread after it got bumped back to the top. Chad, you've come a long way with this saw, and it looks like you are almost there.

I don't have the specs for the 880, but turn your idle screw clockwise to speed up the idle a bit and set your L screw at whatever your shop manual tells you, and adjust the L further from there if you need to.

 

[chad556]

Ok so it looks like i am stuck with more problems.

I started yesterday with both H and L screws out 1 full turn as per the manual for factory settings, The idle speed was set at about 90%. The saw fired up and idled for a little then sputtered out.

I then proceded to turn the L screw out (Left/CCW) about 1/4 turn.

Saw fired up again, sputtered and died even quicker this time. So i went back in with the L screw about a half turn(Right/CW) so now i am at 3/4 turn out from the stop on the low idle screw

Tried to start the saw again and it flooded on me, wouldnt fire at all. Also it started doing that thing where it gets so hard to turn over it pulls the starter cord right out of your hand. I was tired of wrestling with it so i let it sit to dry out overnight.

So that brings me to today, I go to start the saw up and on full choke i could not get it to fire. After 20 or so pulls I look it over and notice blackish oil/gas mix coming out of the muffler, and decomp valve. I pull of the spark plug which is saturated in gas and give the cord a couple pulls to see whats in there. Gas spews out of the spark pulg hole, a lot of gas. So i pull the muffler which has a small puddle of gas in the bottom and flip the saw upside down. Gasoline starts pouring from the carb, exhaust and spark pulg hole. probably almost an ounce of gasoline spills onto the floor.

Now I have flooded things before but this does not seem normal. Could this be another carburetor problem or is it how I have been going about setting the screws on the carb itself?

I currently have the parts off the saw I mentioned and its hanging in my garage with an empty fuel tank hopefully drying. Any advice would be great!

 

[In The Weeds]

After all that you have gone through I don't think bringing it in to be tuned and maybe asking questions would be considered giving up.

 

[chad556]

Yeah its looking that way. I think i will give it one more go once i get it together again and then its off to the dealer to see what they think of it. I have a tachometer coming in the mail so hopefully i can get it to a point where it at least stays running long enough to get some useful data out of it.

For my next attempt i am thinking of starting with the H screw at a half turn and the L screw at 1/4 turn out from the closed position. As i understand it this will make the engine run more lean and cause the rpms to be higher, also leaner running will reduce the flooding issue i hope. Basically i just need it to keep on kicking enough for me to get a screw driver in there and dial in a nice angry sounding idle. From there i will move onto my H screw and hopefully with the help of my new tachometer i can get that tuned into where it needs to be.

Am i going about this the right way? I have never tuned "from scratch" before so how is it done guys?

 

[betterbuilt]

Bump

 

[chad556]

Here is how far I got today:

[video=youtube;f1FOLP_PXFU]http://www.youtube.com/watch?v=f1FOLP_PXFU[/video]

Its is detailed and annotated with how far out from the stops the screws are turned out. Idle screw is set at about 75% all the way through the video though at some point towards the end i do bump it up a bit.

Its close but i still cant get it to run. When i got it to idle for a bit it died right when i picked it up and hit the throttle.

I'm scared to really rev it up because i don't have a clue what this should sound like at its max rpms, just hoping my tach will come in the mail soon. it sounds pretty good in the video but I would hate to blow it up agian.

Also the spark arrestor screen is not in the muffler. Would this make it easier on me if i replaced it?

Would repalcing the hoses maybe make a difference? the fuel line feels pretty stiff to me.

Any of the finer points of carb tuning that i'm lacking here?

Thanks in advance, you guys have been a great help, I wouldn't have gotten this far if it weren't for you!

 

[gink595]

All of the HT carbs I had all of them had a problem and I got them figured out using a pop off tester. Every single one of them had a air leak in a different spot. One was at the fuel inlet elbow, the other had a small defect that wouldn't let the pump diaphram seat against the body of the carb. And the other wouldn't hold seat pressure and would drip fuel. Without that gage I was pulling my hair out.

 

[In The Weeds]

Great thread, got to admire your perseverance. opcorn:

 

[RiverRat2]

All of the HT carbs I had all of them had a problem and I got them figured out using a pop off tester. Every single one of them had a air leak in a different spot. One was at the fuel inlet elbow, the other had a small defect that wouldn't let the pump diaphram seat against the body of the carb. And the other wouldn't hold seat pressure and would drip fuel. Without that gage I was pulling my hair out.

I agree with Gink595,,,,I think If you dont have a pressure tester,,, Take it to the shop,,, I check them all the time,,, lay it on its side like you gonna fuel it open gas tank, I have a wire hook that I made and fish the pick up body out of the tank, leave the hook behind the big rib on the fuel line just before the pick up body, pull the pick up body off, insert the hose bib fitting on the tester and pressure it up,, it should hold about 4.5 bar or around 16 psi indefinitly...

Im just curious,,, if it is flooding that is usually a stiff metering diaphragm or a fuel inlet lever setting that is too high or a inlet valve anomoly,,,,, JMHO,,,,,I dont think trying to adjust the carb by leaning it out is the answer,,,

 

[blsnelling]

I believe your L is still too lean. Back it out another 1/4 turn. Once it's running, slowly turn it back in. You'll hear the idle increase. At some point, it will stop increasing and just want to die. Back it back out 1/8-1/4 turn. Fine tune for throttle response.

You'll have to tune the H needle in the cut since the saw is rev limited. You want it to 4-stroke when you let off the load, than then clean out with a load.

 

[chad556]

Thanks for all the responses guys! Hopefully I will have time to play around with it tomorrow. I am hoping it is just an adjustment that I need and the carb doesnt need work. If i have to take it in then i guess thats that though...

That pump diaphram that i used was the best of the three i had. One was stiff and brittle like a dry leaf but the other two were in pretty good shape. Once I get that idle solid, and it doesnt cut out when i hit the gas I think I can finally call this project a done deal.

 

[In The Weeds]

When you are all done I would be interested to know how much you have into the saw altogether to date?