Stihl 041 Rebuilt - Running too rich

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Patrick1903

Patrick1903

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Hello. I have an old 041 that’s been in the family for 40 plus years. I got it a year ago with a scored piston. It had been sitting a couple years prior. I put a piston/cylinder kit in it. Put original carb back on (after cleaning), and it started right up. Got back to it a month later or so and cut some wood with it. It ran fine for 10/15 mins, then died. Not hit on restart attempts. Plug was fouled. Replaced plug, fired right back up. Same thing, 10 mins later, plug fouled and wet. It’s also really making a mess when running (see picture). At first I thought it was bar oil but it’s pretty black and I see it around the exhaust. Also will get some darker smoke from the exhaust. Seems like it’s really rich or getting way too much fuel. I bought a replacement carb because it seemed like it was flooding, but same issue. I should note, the first time it died, i cranked for awhile with no result. I pulled plug, it was wet. There was a lot of gas in the cylinder. And seemed like it was coming from exhaust too. Really wet around exhaust. I dried it out and replaced plug, fired right up.
Summary of what I have done:
- New piston/cylinder (https://www.hlsproparts.com/Stihl-041-cylinder-and-piston-assembly-44mm-p/h30041.htm)
-New plugs (handful of them!)
-New air filter
-Cleaned old carb
-Replaced original carb with new (cheaper Chinese carb)
-Tuned carb
-Points replaced with chip 15 years ago
-Fuel was 25:1 for break in, then after the first couple plugs switched to 40 or 50:1. Seemed the same. Never really completed a break in since I can’t keep it running long enough.

The saw has plenty of torque when you open the throttle, accelerates smoothly, cuts well and runs well under load, then after 10-15 mins will just die out, seemingly overcome with richness (seems to happen when idled down between cuts, but not necessarily long periods of idling).

I’d love to be able to keep the saw alive…so what are your thoughts? Thanks! Patrick
 

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Vintage Engine Repairs
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I love the sound of the 041’s they are wonderful.

Firstly and most importantly, did you discover the cause for the scored piston?

Secondly did you pressure and vacuum test the fuel line, impulse line, carb and crank case?

I’d suggest firstly cleaning the original carb and getting an OEM kit for it along with a new fuel filter.

Chinese ones are very hit and miss and if yours wasn’t run with the newer ethanol fuel it is quite possibly in good condition be it a bit dirty.

I’d hazard a guess and say that the CDI unit that was installed is possibly causing you issues.

Are you able to upload a short video of it starting and running?
 
Patrick1903

Patrick1903

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Firstly and most importantly, did you discover the cause for the scored piston?
No. But when I took the saw out of the barn (had been sitting 3 to 5 years?) the exhaust was loose, and it hadn’t been intentionally pulled apart. I was wondering if perhaps debris got in there (especially since it bolts on to cylinder and faces skywards, seems like it would be easy for debris to sit around the port). The ring was welded into the piston. Cylinder actually seemed in good condition.
Secondly did you pressure and vacuum test the fuel line, impulse line, carb and crank case?
Yes, I put new impulse and fuel lines on it. I don’t recall if I changed the filter. Pressure/vac tested crankcase. Slow leakage over a couple mins, but I thought that didn’t seem too bad. I didn’t see bubbles from crankshaft either side. I do not think I tested the new carb. That was probably because it was acting up on old carb, so just went for the swap to see if it would change/fix it.
I’d suggest firstly cleaning the original carb and getting an OEM kit for it along with a new fuel filter.
Initially I had pulled the original carb apart, cleaned with carb spray and air, got a Stens gasket and diaphragm kit, wrong one. I was able to use the diaphragm on fuel pump side but not metering side. Original Metering side was more pliable then I thought it would be, so I reused it. I was going to buy a new kit, then figured I’d try the cheap carb first after experiencing the problems with the original carb.
Chinese ones are very hit and miss and if yours wasn’t run with the newer ethanol fuel it is quite possibly in good condition be it a bit dirty.
I knew the Chinese one would be a crapshoot…and as I said above, I was surprised the diaphragm on original wasn’t really stiff. I was considering using the metering diaphragm from the replacement carb on the original carb since I never had replaced the metering diaphragm.
I’d hazard a guess and say that the CDI unit that was installed is possibly causing you issues.
I was wondering if somehow spark could be an issue - not strong/hot enough. Was considering if I should try to replace that.
Are you able to upload a short video of it starting and running?
I have a 25 sec vid of it right after I finished the rebuild and started it. So keep in mind I hadn’t tuned it (didn’t have bar and chain there) and hadn’t run for more than 30 seconds or so at a time. It was a month later that I finally cut with it and then realized it was fouling plugs. The saw is in the barn, 2 hours from me. I wish I had videos of it running at power, good throttle response, etc, I’ll have to get that next time, (may be a couple weeks or more before I get back).
Additional photos and video of electronic ignition (Atom) module, exhaust port, scored piston/ring - here:

 
Vintage Engine Repairs
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Ahh you’re running the ATOM CDI. I would say that is very likely one of your issues if it craps out after it’s warm. It’s very much a sign of a poorly functioning capacitor. I had the same ATOM CDI failing on my Stihl 07, so I converted it back to points and it was great.

Na a slow leak like that isn’t your problem.

I’d say swap the carbs back for now it just
 
Vintage Engine Repairs
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Ahh you’re running the ATOM CDI. I would say that is very likely one of your issues if it craps out after it’s warm. It’s very much a sign of a poorly functioning capacitor. I had the same ATOM CDI failing on my Stihl 07, so I converted it back to points and it was great.

Na a slow leak like that isn’t your problem.

I’d say to swap the carbs back to OEM.

try this next time:

What plus are you using? OEM recommended plug or Chinese? If not get a genuine NGK set a camera up and video it cutting (only need a few seconds under load to see if it’s too rich)

and continue to run it till it dies (when it dies don’t keep trying to start it again) then take plug out and test for spark. no spark? Try new plug NGK (or Bosch) (have to do it literally within seconds) if that doesn’t spark then the cdi is most likely your issue.
 
Patrick1903

Patrick1903

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Ahh you’re running the ATOM CDI. I would say that is very likely one of your issues if it craps out after it’s warm. It’s very much a sign of a poorly functioning capacitor. I had the same ATOM CDI failing on my Stihl 07, so I converted it back to points and it was great.
OK, that's interesting. Would it be easier to switch to another chip? I had read on this forum that there can be issues with certain chips due to 4 magnets inside the flywheel. The old points were worn out I believe, so that's why it was swapped to the ATOM chip years ago.
try this next time:

What plus are you using? OEM recommended plug or Chinese? If not get a genuine NGK
It has an appetite for everything. NGK, Bosch, and Champion. They were all the correct plugs, and gapped to .020.
set a camera up and video it cutting (only need a few seconds under load to see if it’s too rich) and continue to run it till it dies (when it dies don’t keep trying to start it again) then take plug out and test for spark. no spark? Try new plug NGK (or Bosch) (have to do it literally within seconds) if that doesn’t spark then the cdi is most likely your issue.
It'll be a few weeks before I can get back to the saw - really wish I would have thought to record a video of it under load. It really seemed like it was doing great, plenty of power in the cut, then the pretty sudden die-out. I had to double check that I hadn't bumped the switch to "off."

Thanks for the suggestion on the immediate plug swap. Great idea. Sorry for the delay to test this - like I said, probably 2-3 weeks before I'm back to the saw.
 
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Atom no longer sell that specific unit, though I’m sure you can find an alternative. Correct many (most?) CDI’s won’t work with a 4 magnet flywheel, though I’m sure you’ll find one suitable if you want to stick with that route?

Dieing like that after 10-15 mins is almost certainly capacitor related.

Running very (VERY) rich can also cause the same issue, yes. If it’s literally blubbering constantly (4 stroking in the cut the whole) that’s the second thing to consider OR long periods of constant idling.
Though for the former you’d be down on power and it would be slowww going in the cut. If you have an idea of tuning I’d say you weren’t likely running it that rich.

If swapping the plug right away fixes it and you get spark back then it’s most likely your tune , if it doesn’t, and you have to wait a few mins for spark to return with the new clean plug, it’s the CDI.

A video is key here :)

No rush, I look forward to journeying with you on this one and we’ll get to the bottom of it.

let’s hope it’s the plug for simplicity!
 
Patrick1903

Patrick1903

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If swapping the plug right away fixes it and you get spark back then it’s most likely your tune , if it doesn’t, and you have to wait a few mins for spark to return with the new clean plug, it’s the CDI.

A video is key here :)

No rush, I look forward to journeying with you on this one and we’ll get to the bottom of it.

let’s hope it’s the plug for simplicity!
Merry Christmas all. I finally got back to the saw. I wasn’t able to accomplish the experiment of quickly swapping plugs when the saw died. But I was able to get a video of the saw running and also examine plug right after it died this time. After about 2.5 mins, the starter rope caught the flywheel and killed the saw. Broke the starter rope. The culprit appears to be 2 of 3 missing screws on the flywheel cover side. I thought I knew I’d 1 missing screw, but apparently the other one vibrates out at some point. The saw does that with screws in the handle, exhaust, everywhere. I need to check all of the screws and use thread locker on them. I’m suspecting the crank on the clutch side is slighly bent and making the saw vibrate more than it should. Notwithstanding my mess up with the screws on the flywheel cover, back to the original issue.

Here were the carb settings. This is in the replacement Nimtek carb I bought when I thought there was a carb flooding issue with the Tillotson carb. (Turns out both carbs were seemingly doing the same thing). I am intending to get the tillottson back on there.

L 1 1/4 (1.25)
H 3/4 (.75)

Here is a video with me running the saw at WOT then into the wood. It seemed pretty good, but would seem to idle down too low, so I had to feather it to keep it running. Figure I can adjust the idle adjustment later.

I pulled plug and show that in the album as well. The old plug that was on there the last time it died is on the right (fouled). The one I was running when the saw abruptly died is on the left. Doesn’t at all look like fouling was happening when it was running at power.

https://photos.app.goo.gl/1diXL4oKub8pWQ
 
Vintage Engine Repairs
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Merry Christmas all. I finally got back to the saw. I wasn’t able to accomplish the experiment of quickly swapping plugs when the saw died. But I was able to get a video of the saw running and also examine plug right after it died this time. After about 2.5 mins, the starter rope caught the flywheel and killed the saw. Broke the starter rope. The culprit appears to be 2 of 3 missing screws on the flywheel cover side. I thought I knew I’d 1 missing screw, but apparently the other one vibrates out at some point. The saw does that with screws in the handle, exhaust, everywhere. I need to check all of the screws and use thread locker on them. I’m suspecting the crank on the clutch side is slighly bent and making the saw vibrate more than it should. Notwithstanding my mess up with the screws on the flywheel cover, back to the original issue.

Here were the carb settings. This is in the replacement Nimtek carb I bought when I thought there was a carb flooding issue with the Tillotson carb. (Turns out both carbs were seemingly doing the same thing). I am intending to get the tillottson back on there.

L 1 1/4 (1.25)
H 3/4 (.75)

Here is a video with me running the saw at WOT then into the wood. It seemed pretty good, but would seem to idle down too low, so I had to feather it to keep it running. Figure I can adjust the idle adjustment later.

I pulled plug and show that in the album as well. The old plug that was on there the last time it died is on the right (fouled). The one I was running when the saw abruptly died is on the left. Doesn’t at all look like fouling was happening when it was running at power.

https://photos.app.goo.gl/1diXL4oKub8pWQ
Video doesn’t work
 
gumneck

gumneck

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Have you tried upping the idle about an eighth of a turn at a time? That's simple to see if you can get it to idle without constantly blipping the trigger? I would also open the low a little at a time as well if it falls flat when hitting the trigger. If all is well otherwise, it should idle with all you've checked and as good as it runs WOT.
 
Patrick1903

Patrick1903

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My guess is the ignition module is your issue with it cutting out.

Side note - Those old saws like the 041 have a low max rpm and yours is screaming. Your chain looks dull and dry too?

Yep as you say, turn the idle up a little :)
Ok thanks everyone. I’ll try to get this thing back running with replacement starter rope, replace the hardware (with loctite blue on the ones backing out).

Regarding ignition, what would any of you recommend regarding replacing the old Atom CDI module? I have done some research and seems like for most electronic modules I’d have to replace the flywheel since they’re not compatible (I believe that was referenced earlier on this thread, also.)

Regarding the chain, I had just grabbed the bar/chain off another saw. I wasn’t even looking at how it was oiling while it was running since I was concentrating on keeping it cutting. I was anticipating it running to a point and then dying out (though not the way it died this time.). I had the fuel and bar oil tanks filled when I ran it. I also figured it would oil ok because the saw spews oil when it’s idling with bar off.
 
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Ok thanks everyone. I’ll try to get this thing back running with replacement starter rope, replace the hardware (with loctite blue on the ones backing out).

Regarding ignition, what would any of you recommend regarding replacing the old Atom CDI module? I have done some research and seems like for most electronic modules I’d have to replace the flywheel since they’re not compatible (I believe that was referenced earlier on this thread, also.)

Regarding the chain, I had just grabbed the bar/chain off another saw. I wasn’t even looking at how it was oiling while it was running since I was concentrating on keeping it cutting. I was anticipating it running to a point and then dying out (though not the way it died this time.). I had the fuel and bar oil tanks filled when I ran it. I also figured it would oil ok because the saw spews oil when it’s idling with bar off.
I’d suggest returning to points and condenser. Keep it original. It will be more valuable if you ever sell it and they are very reliable.

Re the oiling, I presume the hole is the bar is probably not aligning with the oil galley.

I’d still say to check that skirt through the exhaust port.
 
Patrick1903

Patrick1903

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Have you tried upping the idle about an eighth of a turn at a time? That's simple to see if you can get it to idle without constantly blipping the trigger? I would also open the low a little at a time as well if it falls flat when hitting the trigger. If all is well otherwise, it should idle with all you've checked and as good as it runs WOT.
I’ll do that, thanks.
 
Patrick1903

Patrick1903

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Any idea what numbers your compression is before and after the cylinder swap?
No, sorry. I will try to do a compression check on it once starter is fixed. I had a cheap compression tester when I first started looking at the saw. The numbers didn’t make any sense. (My brand new saw showed 60psi.) And after seeing the scored piston and welded ring I wasn’t too worried about what the accurate number was. (I subsequently bought a mityvac tester that seems good. My newer saw shows about 155psi cold.)
 
Patrick1903

Patrick1903

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I’d suggest returning to points and condenser. Keep it original. It will be more valuable if you ever sell it and they are very reliable.

Re the oiling, I presume the hole is the bar is probably not aligning with the oil galley.

I’d still say to check that skirt through the exhaust port.
This project has been slightly delayed. I’ll try to get a look at it soon.
1) What’s a good source for points and condenser. I saw one on Amazon through Little Red Barn - would that be an ok replacement?
2) Regarding piston skirt, what would you be looking for there?
 
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