Stihl ms 211/C What could be wrong with this saw?

[1964johnr]

I have spent a lot of time and a little money trying to solve this puzzle without success. I'm hoping someone can look at the clues and find the issue. The saw has light use and a service kit at the end of each log burning season and has been working well for 8 years, until now. A few weeks ago it started running rough, then it was difficult to start and cut out after a minute and now it will sound like it will start when I pull on the starter chord, but it wont fire up at all. This is what I have done so far.
Tried two different spark plugs and two different fuel filters.Drained the fuel and refilled with new 50/1 mix. New air filter. New oil filter.
Holding the saw by the starter handle at waist height it drops, then holds, then drops a little more and holds and drops again, so I think it has compression.
I stripped it down and found a layer of carbon on the top of the piston and the top of the cylinder. The cylinder top is also pitted all over the surface, like little pin pricks. I cleaned all the carbon off but the pitting remains. The piston had one or two vertical scores on it but the rings look good and move freely.
I took the carb out and took top and bottom plates off. No muck inside, but the small circular screen beneath the bottom plate was a little clogged, so I took it out and cleaned it and put it back.
Visual inspection of the fuel line without removal looks good, no fuel leak. 
New gasket sealer for the cylinder base, oiled all of the piston parts before reassembling and put new crank bearing seals in.
Reassembled, pulled it over with the choke out then switched to normal start. It fired up, ran rough for a minute while spitting out large amounts of white smoke and then conked out. Tried starting it again, but no luck. When it did start the first time, the throttle didn't have any effect, it wasn't working. I have no idea what to try next. Does anyone have any ideas?

[Old Greenhorn]

You have tried everything I would have looked at, and then some. The only thing that pops into my head (and I don't know this saw model) but could there be an air leak between the carb and the cylinder? If not that, then I would turn my attention to the carb for a little more in depth work.
Good Luck and keep us posted.

[Jack S]

Don't forget to check for a restricted muffler.  Carbon buildup happens over time 

[Hilltop366]

Fouled the plug with all the oil would be my guess.

[dougand3]

All good ideas above. 
After pulling 10x and no start...is plug wet or dry?
Ring end gap? Rings can look good but be worn, decreasing compression. Not worth pulling jug again, tho.
Tiny passages under welch plug in carb varnished? This carb is $15 online. When that cheap, I don't bother with a kit.

[1964johnr]

The plug is wet with oil when I pull it over a few times. I had the carb out today and stripped it down quite a long way. I squirted carb cleaner down all of the holes and past the butterflies before reassembling. The muffler is a one peice unit with no screen in it. The inside was covered with carbon deposits, so I cleaned it up as best I could. After doing these two things I fired it up and it ran, all be it erratically for a few minutes. The saw had a lot of vibration and the cylinder and muffler got extremely hot very quickly. I had the top cover off when I pulled it over and when I placed the palm of my hand just above the cylinder I could feel air coming up like if you put your hand in front of a hair dryer. Not sure if this is normal, or could it indicate a major air leak, perhaps I missed it when I checked the intake boot. While it was running, I pulled on the throttle to see what effect this had. It tried to throttle up but got bogged down and then conked out.

[zimraphail]

I used to repair saws small engines etc on the side! First thing i do is pull a cold compression test. if it passes I check spark, if hot blue spark i will then pull carb if it doesnt run! I use carb cleaner but pull apart and also blow out orifices with compressesd air! If a diaphram carb the is normally a small inlet screen in fuel inlet . This i sprat and blow out as well often they are plugged! Be carefull pulling apart you may need a carb kit?

[dougand3]

The plug is wet with oil when I pull it over a few times

Sounds like flooding. Did you clean needle and seat? Adjust lever? Sometimes seat is damaged and real hard to fix. This would be much simpler to get a $15 carb online.

[Hilltop366]

It depends on how much oil you put in there when reassembling the saw, it needs very little. If you think about how little oil the saw actually gets when running with the tiny bit of gas oil mix that goes in the engine on each stroke and the ratio of oil in the gas, it is a tiny amount. So if the engine has even a tea spoon of oil in it will have more oil than gas in it and it is going to billow smoke for a bit until all the access oil is washed out of the engine and it will most likely foul the plug again.

That being said the carb has been apart and needs to be adjusted but it is difficult to set it up until you can get rid of the access oil and get it warmed up, if it bogged when trying to accelerate from low speed turn the low speed adjust out a bit and try again. Accelerating slowly may help get the rpm's up and clean out the engine without fouling the plug. 

The engine is fan cooled from the flywheel fins so it needs to have the shroud in place to direct air around the cylinder and head to prevent over heating.

[motzingg]

I have 3 pieces of equipment that do something similar right now-  a BG86, BG85 and an FS75

something in the carb- either check valve, unloading valve, ???  is causing them to spray tons of fuel into the engine.  they will start (cold/dry) and run for a few seconds and by that time they have dumped a ton of fuel, its spraying out the exhuast, and they wont start. 

Come back a day or ? later and they might fire right up and run perfect, they might do the same thing again. 

I'm not one to just throw in the towel and pop on a whole new carb, but on another forum someone had posted there is a small check valve permanently installed in the carb that can go bad causing this?  I've read tons on forums and tech articles about these Zama carbs and can't figure out if there is a way to fix it.  The FS75 i'm just fixing to sell so it will probably be the test dummy and get a new carb, and see if that fixes it.  Gaskets, diaphragms, all that stuff is fine. 



Anyhow... that might be your issue. For me (and in all three pieces of equipment) the issue is maddeningly intermittant.  The BG86 hasn't acted up in the last 50+ starts, the FS75 trimmer only does it about 1 out of every 4 starts, but when it does it, you're screwed. Crack a beer and wait till tomorrow because that sucker is flooded. 

[btulloh]

Mis-adjusted floats have caused these types of problems for me in the past. Good to check, just to rule out one more thing at least.

[motzingg]

Exactly, but seeing as there are no floats on these carbs... 

I did free up and clean them, checked the diaphragms, checked the springs, lever doohickey, all that stuff that serves as the 'float bowl/spring' arrangement. 

But yes, to your point (or what i think your point is?) my guess is that its something in the fuel supply regulation system.  One article i read talks about a very small clear plastic check valve disc that is part of the pressed in main jet assembly.  If that is the problem, it would explain a lot. 

[luap]

Exactly, but seeing as there are no floats on these carbs...

I did free up and clean them, checked the diaphragms, checked the springs, lever doohickey, all that stuff that serves as the 'float bowl/spring' arrangement.

But yes, to your point (or what i think your point is?) my guess is that its something in the fuel supply regulation system.  One article i read talks about a very small clear plastic check valve disc that is part of the pressed in main jet assembly.  If that is the problem, it would explain a lot.

The diaphragm with the metal disc acts as the float and engages the lever with spring that operates inlet needle valve. The carb manufacture will specify the height the lever is set at. It is also easy to put the diaphragm in upside down and to get the gasket on wrong side of diaphragm. both will contribute to wrong amount of gas fed in. wrong setting of hi lo screws may also contribute to your problem. If it starts and runs at all then I would start there. You are right about flooding, may as well go do something else for awhile, lean it out and try again