Stihl 021 - need help with air leak(?) symptoms

[HakonJ]

Hi, all

I have an old Stihl 021 after my grandfather that I'm trying to revive. It didn't start when I got it. I tore it down and gave it a new fuel line (old one was torn), fuel filter, spark plug, air filter and breather tube. After this, I got it running, but not well. It idles just fine after a cold start, and I can cut wood with good power for a few minutes. However, after it gets warm, it revs up if I tip it bar-side down (excessively, chain running away at high speed). If I tip it back, it idles down. Sometimes it idles down to a halt. It's fairly easy to restart, but sometimes I do have to give it some throttle while pulling the starter. After it gets warm and starts behaving like this, it just bogs down if I try to cut wood.

After this, I suspected the crank seals needed replacement, so I did that (disassembled everything, splitting the clam-shell, reinstalling seals and making a new gasket). It seemed to work fine for a short while, but soon ran into the same problem. I thought maybe I did a bad job with the gasket, and tried redoing that once more, with no luck.


How would you proceed from here? My gut feeling (which isn't worth much) still tells me it's an air leak somewhere.

Could this be an air leak related to the carburetor? Or an issue with the carb itself?

Could my gasket material be the culprit? It's silicone based, rated for 300 C (~600 F), but not Dirko.

Could my method for applying the gasket be it? I applied sparingly, scraped off the surface with a razor blade, put it together and carefully reinstalled it into the housing. Tightened the bolts evenly until gasket material started appearing at the edge, waited for 1 hour and then tightened to spec (as per instruction). How would you do it?

Coil issue seems unlikely, as the symptoms are related to the saw's orientation.

 

[lone wolf]

Hi, all

I have an old Stihl 021 after my grandfather that I'm trying to revive. It didn't start when I got it. I tore it down and gave it a new fuel line (old one was torn), fuel filter, spark plug, air filter and breather tube. After this, I got it running, but not well. It idles just fine after a cold start, and I can cut wood with good power for a few minutes. However, after it gets warm, it revs up if I tip it bar-side down (excessively, chain running away at high speed). If I tip it back, it idles down. Sometimes it idles down to a halt. It's fairly easy to restart, but sometimes I do have to give it some throttle while pulling the starter. After it gets warm and starts behaving like this, it just bogs down if I try to cut wood.

After this, I suspected the crank seals needed replacement, so I did that (disassembled everything, splitting the clam-shell, reinstalling seals and making a new gasket). It seemed to work fine for a short while, but soon ran into the same problem. I thought maybe I did a bad job with the gasket, and tried redoing that once more, with no luck.


How would you proceed from here? My gut feeling (which isn't worth much) still tells me it's an air leak somewhere.

Could this be an air leak related to the carburetor? Or an issue with the carb itself?

Could my gasket material be the culprit? It's silicone based, rated for 300 C (~600 F), but not Dirko.

Could my method for applying the gasket be it? I applied sparingly, scraped off the surface with a razor blade, put it together and carefully reinstalled it into the housing. Tightened the bolts evenly until gasket material started appearing at the edge, waited for 1 hour and then tightened to spec (as per instruction). How would you do it?

Coil issue seems unlikely, as the symptoms are related to the saw's orientation.

Well you should get a vac - pressure tester like a Mitivac 8500 . Then seal off the intake and exhaust ports and test it at the impulse line ,use soapy water in a spray bottle to look for leaks .

 

[MartDalb]

IF, and..I just shoot this one because I have seen it once. (it is more likely to be an air leak)
Check the carburator springs. Do they spring back?

It is a long shot, but I had this Stihl MS211C, hardly used, serviced etc.
Ran fine, started, but if you tilted it, up or down, sideways, it would do weird things.....

turned out the spring to swing the "choke shutter" was broken off a tad and any movement would make the saw do funny stuff.

See picture.

 

[Vintage Engine Repairs]

Well you should get a vac - pressure tester like a Mitivac 8500 . Then seal off the intake and exhaust ports and test it at the impulse line ,use soapy water in a spray bottle to look for leaks .

100%

OP - Without actually testing it, there will be lots of guessing and money thrown at it without success and still not knowing what’s causing the problem.

 

[HakonJ]

Well you should get a vac - pressure tester like a Mitivac 8500 . Then seal off the intake and exhaust ports and test it at the impulse line ,use soapy water in a spray bottle to look for leaks .

Short version: read bold text.

Unfortunately I can't justify buying equipment like that for the very few occasions I'll do pressure/vac-testing. I fabricobbled a pressure tester from what I had laying around. Basically a pressure gauge connected to the impulse line and my air compressor. I managed fairly well to stay at or below 0.5 bar for testing.

I tested this in three ways, and I believe I have narrowed it down to the intake boot (let me know if you think my conclusions are wrong). First I tested with the intake boot removed, sealing the intake port on the cylinder directly. It showed no leak (at least it held constant pressure for 20 seconds before I could see a tiny movement in the gauge). I also tested the way you can see below, with the intake sealed at the carburetor mount on the orange plastic piece. This way I also got to test the intake boot itself. It leaked noticably at the cylinder-side of the boot, i.e. where the intake boot is seated on the cylinder intake.



Now, I know the baffle plate that sits on the boot (see image below) is missing on the image above. I assume this part makes the boot seat/seal better against the intake itself, so I also tested with this part installed. However, I still got air leaks at the base of the boot.



At my local dealer, the OEM intake boot costs about $32. In the aftermarket, I can buy this boot, impulse line, an entire carb, air filter, fuel filter +++ for about $15. My experience with AM parts is not particularly good, but that OEM price is not exactly tempting.

Also, how important is that baffle plate? I thought about simply removing it and secure the existing intake boot with a hose clamp, like I know it's done on some other Stihl models. Alternatively, modify the baffle plate to make room for a hose clamp.

What would you do in this situation?

IF, and..I just shoot this one because I have seen it once. (it is more likely to be an air leak)
Check the carburator springs. Do they spring back?

It is a long shot, but I had this Stihl MS211C, hardly used, serviced etc.
Ran fine, started, but if you tilted it, up or down, sideways, it would do weird things.....

turned out the spring to swing the "choke shutter" was broken off a tad and any movement would make the saw do funny stuff.

See picture.

Thanks for the tip. I just checked, and the springs are working well.

OP - Without actually testing it, there will be lots of guessing and money thrown at it without success and still not knowing what’s causing the problem.

You're absolutely right, thanks!

 

[lone wolf]

Short version: read bold text.

Unfortunately I can't justify buying equipment like that for the very few occasions I'll do pressure/vac-testing. I fabricobbled a pressure tester from what I had laying around. Basically a pressure gauge connected to the impulse line and my air compressor. I managed fairly well to stay at or below 0.5 bar for testing.

I tested this in three ways, and I believe I have narrowed it down to the intake boot (let me know if you think my conclusions are wrong). First I tested with the intake boot removed, sealing the intake port on the cylinder directly. It showed no leak (at least it held constant pressure for 20 seconds before I could see a tiny movement in the gauge). I also tested the way you can see below, with the intake sealed at the carburetor mount on the orange plastic piece. This way I also got to test the intake boot itself. It leaked noticably at the cylinder-side of the boot, i.e. where the intake boot is seated on the cylinder intake.

View attachment 908294

Now, I know the baffle plate that sits on the boot (see image below) is missing on the image above. I assume this part makes the boot seat/seal better against the intake itself, so I also tested with this part installed. However, I still got air leaks at the base of the boot.

View attachment 908298

At my local dealer, the OEM intake boot costs about $32. In the aftermarket, I can buy this boot, impulse line, an entire carb, air filter, fuel filter +++ for about $15. My experience with AM parts is not particularly good, but that OEM price is not exactly tempting.

Also, how important is that baffle plate? I thought about simply removing it and secure the existing intake boot with a hose clamp, like I know it's done on some other Stihl models. Alternatively, modify the baffle plate to make room for a hose clamp.

What would you do in this situation?


Thanks for the tip. I just checked, and the springs are working well.



You're absolutely right, thanks!

I would put grease on the boot for a tighter seal the pressurize it and dunk it in a bucket of water and note all the leaks.

 

[SteveSr]

At my local dealer, the OEM intake boot costs about $32. In the aftermarket, I can buy this boot, impulse line, an entire carb, air filter, fuel filter +++ for about $15. My experience with AM parts is not particularly good, but that OEM price is not exactly tempting.

AM parts, especially rubber, aren't known for holding up to fuel and vapor.

Also, how important is that baffle plate? I thought about simply removing it and secure the existing intake boot with a hose clamp, like I know it's done on some other Stihl models. Alternatively, modify the baffle plate to make room for a hose clamp.

Unless the manifold boot is cracked or torn it should be reusable. Is it still stiff but flexible? Make sure that the surfaces are clean and add a light coat of grease to ease assembly and sealing.

And, yes, that baffle is important for directing airflow... AND holding the boot on the cylinder.

 

[HakonJ]

AM parts, especially rubber, aren't known for holding up to fuel and vapor.


Unless the manifold boot is cracked or torn it should be reusable. Is it still stiff but flexible? Make sure that the surfaces are clean and add a light coat of grease to ease assembly and sealing.

And, yes, that baffle is important for directing airflow... AND holding the boot on the cylinder.

So I will stay away from am parts. The boot is not torn or cracked, but it is the original from 1991. Not nearly as supple as other new OEM rubber parts. Quite stiff in comparison, but still flexible. I replaced a leaking oil tube on a 211 recently. That tube (the old one) felt much more supple and flexible than the boot on the 021.

Will grease help with long term sealing? Any recommended types of grease, or just general purpose grease?

Btw, will I hurt anything by trying the saw without the baffle plate, securing the boot with a hose clamp? I'm thinking as a test, to "solidify" my diagnosis with the boot being the part at fault. If it works well after being warmed up, I know the boot is the likely part to blame.

I will also try the water-dunk test with pressure, to see if it is leaking elsewhere.

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[lone wolf]

So I will stay away from am parts. The boot is not torn or cracked, but it is the original from 1991. Not nearly as supple as other new OEM rubber parts. Quite stiff in comparison, but still flexible. I replaced a leaking oil tube on a 211 recently. That tube (the old one) felt much more supple and flexible than the boot on the 021.

Will grease help with long term sealing? Any recommended types of grease, or just general purpose grease?

Btw, will I hurt anything by trying the saw without the baffle plate, securing the boot with a hose clamp? I'm thinking as a test, to "solidify" my diagnosis with the boot being the part at fault. If it works well after being warmed up, I know the boot is the likely part to blame.

I will also try the water-dunk test with pressure, to see if it is leaking elsewhere.

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After the water test you will know a lot more I wouldnt doubt you have other leaks. If you want to do a nice job buy a new boot if there is any question as to it being good.

 

[SteveSr]

So I will stay away from am parts. The boot is not torn or cracked, but it is the original from 1991. Not nearly as supple as other new OEM rubber parts. Quite stiff in comparison, but still flexible. I replaced a leaking oil tube on a 211 recently. That tube (the old one) felt much more supple and flexible than the boot on the 021.

Will grease help with long term sealing? Any recommended types of grease, or just general purpose grease?

Btw, will I hurt anything by trying the saw without the baffle plate, securing the boot with a hose clamp? I'm thinking as a test, to "solidify" my diagnosis with the boot being the part at fault. If it works well after being warmed up, I know the boot is the likely part to blame.

I will also try the water-dunk test with pressure, to see if it is leaking elsewhere.

Long term you can't run without the baffle plate. Hose clamp test would be meaningless. If you still have a leak there with the baffle plate firmly installed you need a new manifold.

The Stihl 1123 series of clam shell motors are known to leak from the pan seal and at that age possibly the crank seals as well.

 

[HakonJ]

Long term you can't run without the baffle plate. Hose clamp test would be meaningless. If you still have a leak there with the baffle plate firmly installed you need a new manifold.

The Stihl 1123 series of clam shell motors are known to leak from the pan seal and at that age possibly the crank seals as well.

Thanks for the reply. My intention with the hose clamp test was only to confirm the manifold as the culprit, not to find a permanent solution.

The dunk test does the same thing, more or less. I tried it, and there were no bubbles at all coming from the pan gasket or crank seals. (Those were replaced just recently, as they were leaking when I got the saw)

I guess I will have to bite the bullet and buy a new OEM manifold.

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[lone wolf]

Thanks for the reply. My intention with the hose clamp test was only to confirm the manifold as the culprit, not to find a permanent solution.

The dunk test does the same thing, more or less. I tried it, and there were no bubbles at all coming from the pan gasket or crank seals. (Those were replaced just recently, as they were leaking when I got the saw)

I guess I will have to bite the bullet and buy a new OEM manifold.

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Exactly, but you should do a vac test too not just a pressure test to make sure the seals are working 100 percent.

 

[SteveSr]

Thanks for the reply. My intention with the hose clamp test was only to confirm the manifold as the culprit, not to find a permanent solution.

The dunk test does the same thing, more or less. I tried it, and there were no bubbles at all coming from the pan gasket or crank seals. (Those were replaced just recently, as they were leaking when I got the saw)

I guess I will have to bite the bullet and buy a new OEM manifold.

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Back to basics...

1. What carburetor (model) is installed and what have you done to it (i.e. replaced / rebuilt, etc.)? This saw is definitely old enough to need an OEM kit by now. Could also have a load of sawdust between the metering cover and diaphragm which may move when the saw is tilted causing issues.

2. Pull the muffler and post photos of the piston through the exhaust port. Sometimes scored pistons will still run but get wonky after the engine warms up and tolerances increase.

Not too long ago I had another 1123 saw that came from a coastal area with salt air and water. It ran surprisingly well. The bucket leak test found a corrosion leak in the engine pan. The air leak was compensated for by a carb metering diaphragm that was stiff as cardboard!

Another way to look for air leaks is with brake cleaner. Using the straw shoot some near the suspected leak (away from the carb intake) and see if the engine dies or behavior changes.

 

[HakonJ]

Back to basics...

1. What carburetor (model) is installed and what have you done to it (i.e. replaced / rebuilt, etc.)? This saw is definitely old enough to need an OEM kit by now. Could also have a load of sawdust between the metering cover and diaphragm which may move when the saw is tilted causing issues.

2. Pull the muffler and post photos of the piston through the exhaust port. Sometimes scored pistons will still run but get wonky after the engine warms up and tolerances increase.

Not too long ago I had another 1123 saw that came from a coastal area with salt air and water. It ran surprisingly well. The bucket leak test found a corrosion leak in the engine pan. The air leak was compensated for by a carb metering diaphragm that was stiff as cardboard!

Another way to look for air leaks is with brake cleaner. Using the straw shoot some near the suspected leak (away from the carb intake) and see if the engine dies or behavior changes.

It is a Zama carb (but I don't have a model number). I forgot to mention it in the first post, but I also installed a new carb kit earlier. OEM, if I remember correctly.

Piston looks good. Here are two images, with piston at top and bottom.

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[SteveSr]

It is a Zama carb (but I don't have a model number). I forgot to mention it in the first post, but I also installed a new carb kit earlier. OEM, if I remember correctly.

Zamas are known to have issues with sticking main nozzle check valves which is a non-repairable fault and can cause erratic operating symptoms. If it sticks closed it will idle fine all day but won't rev up. If it sticks open it will die (or race) at idle depending on the severity of the leak. I generally replace these with the fully adjustable Walbro WT-215 carb that was also used on this model.

Piston looks good. Here are two images, with piston at top and bottom.

Looks good.

 

[HakonJ]

Zamas are known to have issues with sticking main nozzle check valves which is a non-repairable fault and can cause erratic operating symptoms. If it sticks closed it will idle fine all day but won't rev up. If it sticks open it will die (or race) at idle depending on the severity of the leak. I generally replace these with the fully adjustable Walbro WT-215 carb that was also used on this model.


Looks good.

All right, nice to know. Will look into that carb. Not sure what you mean by fully adjustable, but my carb has all three adjusting screws. I have already ordered the new manifold, so will try that first. I have a suspicion that the one installed is not OEM, and it is leaking anyway.

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[SteveSr]

All right, nice to know. Will look into that carb. Not sure what you mean by fully adjustable, but my carb has all three adjusting screws.

It must be a Zama C1Q-S11 which was discontinued in 1994 so your saw is older than that. The newer Zamas were not fully adjustable. The replacement for the S11 was the Walbro WT-215.

I have already ordered the new manifold, so will try that first. I have a suspicion that the one installed is not OEM, and it is leaking anyway.

All genuine Stihle parts will usually have the Stihl "ST" logo molded into them somewhere and/or a partial part number. You may need a magnifying glass to see it but it should be there somewhere.

 

[HakonJ]

It must be a Zama C1Q-S11 which was discontinued in 1994 so your saw is older than that. The newer Zamas were not fully adjustable. The replacement for the S11 was the Walbro WT-215.


All genuine Stihle parts will usually have the Stihl "ST" logo molded into them somewhere and/or a partial part number. You may need a magnifying glass to see it but it should be there somewhere.

Yes, the saw is from 1991.

I assumed there would be a mark, as I've seen it on other parts. Can't find it on this manifold.

Thanks for all the helpful feedback so far. Will update the thread once the manifold is replaced and the saw tested.

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[HakonJ]

I got the manifold today, and have just reassembled and done some initial testing on the saw. The manifold and baffle plate fit much more snugly compared to what it did with the old manifold. Fired it up and let it run for about 10 minutes to get hot. Revved up now and then. Behaves just fine, no racing or stalling when tilting the saw in different orientations. I assume it is working fine now, but will have to do some serious work with the saw to confirm. Thanks again for all the useful input.

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[MartDalb]

I got the manifold today, and have just reassembled and done some initial testing on the saw. The manifold and baffle plate fit much more snugly compared to what it did with the old manifold. Fired it up and let it run for about 10 minutes to get hot. Revved up now and then. Behaves just fine, no racing or stalling when tilting the saw in different orientations. I assume it is working fine now, but will have to do some serious work with the saw to confirm. Thanks again for all the useful input.

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Is it rare that manifolds go bad, but....it happens. Glad you got it fixed