880 runs rich

[Ketchup]

I have a 2004 stihl 880 that I bought online for milling. It's my only 120cc saw so I have a lot to learn. The saw starts great and idles with a nice burble. BUT it's really sluggish in the cut and seems like it's running very rich. No matter how long I spend with my little screw driver I always end back with the same basic settings. Idle out 2 full rotations, Low out 3/4, and High all the way down (clockwise). If I open the high at all it runs even more sluggishly.
The saw has a modded slot in the muffler and after running a few minutes fuel residue is obvious around the exit ports.
This saw has an unlimited coil, so that is ruled out.
So has anyone had a similar problem? I don't get the impression these big tillotson carbs are this hard to tune. I have a friend saying the carb has a bad check valve (though the saw doesn't flood). Do I rebuild it?
There isn't much mystery in a chainsaw, but the inside of a carburetor has some real wizardry. It's not my area of expertise. Any experiences, thoughts or links are welcome. I've search all over for this, but not gotten much info.
Hope somebody can help me out. Otherwise I may be forced to the stihlership...

 

[scallywag]

High all the way down (clockwise). If I open the high at all it runs even more sluggishly.

I wouldn't run this saw again until you either have a carb kit fitted or try another carb.

 

[Ketchup]

I wouldn't run this saw again until you either have a carb kit fitted or try another carb.

That confirms my instincts. Any guess what is happening?
I do understand it's likely related to ethanol, though I have never run any through the saw. I'm trying to understand the mechanics in the carb that are failing. Am I an idiot for thinking I can do the carb kit myself? I've rebuilt 2 smaller Walbro carbs. They both run. I don't have another good carb for this saw and they're spendy.

 

[alexcagle]

I run into carbs sometimes, that as I'm testing them, they hold pressure great at 10psi. However, as the pressure drops a couple psi from appying vacuum to the venturi (with mouth) for a millisecond, it keeps dropping down slowly instead of a holding the pressure solidly at the point vacuum was halted.
It could be simple things like a weak or incorrect fulcrum spring, a deformed rubber tip of the needle, or a catch in the fulcrum on it's pin from corrosion or a squished fulcrum hole for the pin.

Install a new kit.

If you don't have a tester, buy a cheap harbor freight fuel injector compound tester guage, a couple feet of fuel hose, a vacuum tee, and a remote purge primer.
The longer barb on the primer will provide pressure up to 30psi, and will hold it with it's internal check valve. The short barb will provide at least 1bar of vacuum for testing as well.
Test the carb as removed from the saw first to confirm the problem.

 

[HarleyT]

Have you been into the carb at all?

Could just be a loose welch plug, or similar.

 

[HarleyT]

Or incorrect lever height.........

 

[blsnelling]

There's very little inside that carb. Put a kit in it.

 

[HarleyT]

Well,
a lever and a welch plug.....

 

[IddyBiddy]

rebuild/replace the carb

 

[Ketchup]

The kit is in the mail. I'll follow up when it's back together. Thanks everybody!

 

[Ketchup]

So I tore the carb down yesterday with a friend who knows a lot more than me. Very ugly inside. Especially at the valve diaphragm and intake. Fused rubber, crusty white death. Very bad for a saw still running.
Welch plug and check valve are now suspect. Off to the dealer to source a Check valve. Might be buying a new carb. Definitely replacing my lines and filter as well.

 

[Ketchup]

Well, it took a month but I cleaned the carb up pretty well (dental tools, pipe brushes, razor blades, cocktails of solvents) and replaced everything except the jet, check valve and the Welch plug. A very respected Stihl tech said I didn't have symptoms of those being bad, though I may go back in and switch the plug. I also cleaned the tank as best I could and put in a new fuel line and filter. I left the pulse line, but might swap that with new also.
She's back together and running better! The new carb settings are roughly: LA 2.25, L 1.25, H 0.75 turns out. Still surprisingly low on the High needle and not super responsive to tuning. She idles a touch low now and I want more RPMs but definitely running less rich and faster than before. I think more tuning is in order. Ultimately I would like to port this saw, so I really want to chase out the demons before spending the time.
Questions remain:
Replace the Welch plug? I still suspect it because the factory seal on it came off as corroded jelly. If I replace it, what do I seal the new one with?
Also upon reassembly my pop-off pressure was pretty low. 13 psi holding at 10. Is that too low and if so where do I get a more powerful lever spring?
Thanks for all the insight. I wouldn't have gone this deep without you guys. It has been a cool learning experience so far!

 

[HarleyT]

Welch plug is not a problem, unless is is loose, but the plugged orifices under it would be. Just seal the new one with some nail polish if you replace it.
Can you put up a pic of the area?

 

[HarleyT]

Do not mess with the spring, just make sure the lever is the correct height.

 

[Ketchup]

I'm afraid I'm not too good with pictures. I always forget to take them and now the saw is back together. The plug had a ring of eroded orange jelly that came off early in cleaning. I was thinking that was factory, now I'm thinking maybe it was the PO's nail polish. Someone had definitely been in the carb previously. Both needles came out clean, the body went through 5 cycles in the ultrasonic with straight new WD40 and was then gently blown out. The plug itself looks fine, no obvious corrosion or movement.

I ask about the needle spring because the original is MIA so I put one in from a spare kit my buddy gave me. Who knows what carb it was for, not a big one like this. But the internet says a silver Tilly spring will pop between 12 and 15 psi on the larger jet models. (Not sure what size jet I have.) I'll do some cutting and see what it feels like. If I take it back apart I'll take some photos.

 

[HarleyT]

You need to get the correct spring.

 

[HarleyT]

Here is a bulletin with some info.

 

[HarleyT]

1124 122 3001 Spring
Stihl available

 

[Ketchup]

Wow thanks. I forgot part of the reason I bought this saw was parts can stihl be ordered. I'll get one and report back.

 

[Ketchup]

Update:
I got the proper spring. The other one was way too short.


I also put the old needle lever back in, because the new one seemed way too low.


Unfortunately I have no idea if that helped or not. I pulled the cylinder while I had the saw apart to put in new rings (compression was 135 psi) and found this.

My crank arm is scorched. Both bearing ends have this color. Which is very bad.

So it's just parts in a box until I find a crank. Then I have to split and rebuild.

I'm tempted to put it all together and run it, just to check the carb, but I know better...