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Author Topic: Trouble with a pair of O38's  (Read 827 times)

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Offline JoshNZ

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Trouble with a pair of O38's
« on: October 04, 2017, 06:04:13 am »
I have a pair of O38's here I was hoping to get some advice about before I go down any unnecessary rabbit holes.

The first one is an O38 magnum II, always run flawlessly, always started first pull - IF you used it the day or two before. If you used it over a week ago, you might get it going after 25 pulls. Not great for the arm. If it's been over a month then forget about it, it needs the filter or plug removed and a splash of fuel thrown in there. Once that happens it fires and obviously primes itself and it's good to go, runs great again. It has been looked at by the local experts in town and they ended up giving up on it.

I haven't even removed the cover yet, I have a fair idea what it might be on a generic 2 stroke with/without a diaphragm pump, but I've never actually looked at these stihl engines up closely, so I wondered if this particular symptom screams out any problem for the experts?

The other one is just an O38 and has a melted cylinder shroud with a tin patch on it that burns your hand if you get too close. I was in the shop getting a new chain the other day and asked about a replacement shroud, he said he might have one off a wrecked saw, and he brought out one melted in the same place with the same patch on it haha. So I figure maybe this is a common problem with the older O38s. I've ordered a new genuine shroud but wondered before I put it on is there anything that can be done about it? I don't think it's running overly lean or anything, just seems to be too close to the muffler...

What is the difference between the O38 and O38 magnum 2 by the way?

Many thanks in advance.
Josh

Offline Woodcutter_Mo

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Re: Trouble with a pair of O38's
« Reply #1 on: October 04, 2017, 10:58:05 am »
The biggest difference between those 2, the Magnum is larger displacement. Subbing to see what others say about the other problems for future reference as I have a 038 Super which is right between the 038 and the 038 Mag.
-Sawmill- WoodMizer LT25, Kohler, all manual, old "drag" style WoodMizer sharpener, cat claw single  setter
-Saws- Husqvarna 372XP X-Torque, 455 Rancher X-Torque, Stihl 024AV
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Offline Maine372

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Re: Trouble with a pair of O38's
« Reply #2 on: October 04, 2017, 12:34:51 pm »
038 melted top cover sounds like leaking exhaust somewhere. check that the muffler is intact an not leaking where it seals to the cylinder.

MAGII not starting sounds like crank seals leaking, therefore not creating the vacuum to pull fuel through the system. plenty of other things it could be, but if its running fine otherwise that would be my suspicion . other possibilities are impulse line, plugged fuel filter, plugged tank vent, cracked fuel line, or on some carbs there is a vent hole in the cover for the diaphragm. dust and debris can settle in there and prevent the diaphragm from moving its full capacity, starving the saw for fuel.

you can either just do the crank seals, not a bad idea with a saw that old. or you can find a shop that will pressure/vac test to confirm the issue.

Offline JoshNZ

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Re: Trouble with a pair of O38's
« Reply #3 on: October 04, 2017, 06:21:51 pm »
Great answer thanks I hadn't considered crank seals. I think I have enough crap lying around that I can fab up a leak tester and some plugs, I'll see when I get a chance. It will burn tank after tank ringing logs at WOT without running hot and idles perfectly, never seen it runaway lean or anything. So I would be surprised. Following that I guess I just need to go through the fuel system and check everything, I doubt the local shop has been inside the tank/clunk etc.

And I'll have a closer look at that muffler too that sounds about right.

I did some more poking around after that post and found the Stihl 038 - 61cc, 3.8hp. 038 Super - 66.8cc, 4.4hp. 038 Magnum 72cc, 4.8hp. Incase anyone is interested. I'd have never believed there was 10cc displacement between the saws, they look identical. Whoever drew the short straw always used the 038 because of the shorter bar, so I just bought a 20" bar to bring it up to the magnums playing field, I wonder if it was a mistake.

Offline joe_indi

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Re: Trouble with a pair of O38's
« Reply #4 on: October 05, 2017, 05:13:13 am »
Maine372 has covered the possible causes, but include a possible leak in the cylinder gasket also in the list.

Offline Al_Smith

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Re: Trouble with a pair of O38's
« Reply #5 on: October 05, 2017, 07:30:56 am »
FWIW I have a modified Brazilian made 038 Mag that takes up to 10 pulls on a cold start if it hasn't been started for a while but will go on one the rest of the day .I always assumed it to be the carb but it runs like a top so I'm not going to do anything to it .It does have a D-handle though,modified high compression . As a matter of record on a "dry" carb it often takes 6 or more pulls just to fill the fuel chamber in the carb .Some "leak down" and some don't .
That melting the shrouding could be a number of things,no gasket,no directional shrouding on the muffler discharge ports .A stock muffler can cause a lot of heat build but modification of same is another subject .
The main difference I've seen comparing the mag to the AV model other than bore size is the cylinder /piston configuration .The Mag is a flat top piston ( stock) ,center combustion chamber .The AV is a hemi domed piston with an offset combustion chamber .Other than the afore mentioned as far as I know the rest of the parts are interchangable .Some have adapted an 038 Mag cylinder to an AV crankcase with success ,I won't do it .

Offline HolmenTree

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Re: Trouble with a pair of O38's
« Reply #6 on: October 05, 2017, 07:51:26 am »
The choke for the 038 carb is built into the air filter, with age they don't seal properly and don't do their job.
Also the shutter spring weaken and the choke will partially close at WOT causing a over rich condition.

Try putting the 038AV air filter on the Magnum and I pretty well guarantee it will fix your starting problem.
Making a living with a saw since age 16.

Offline Al_Smith

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Re: Trouble with a pair of O38's
« Reply #7 on: October 05, 2017, 02:18:37 pm »
Some of this model used a Bing carb and some I think a Tillotson .Neither had a choke as part of the carb and as been mentioned it was in the air filter .Until the last few years parts for the Bing were rare and to get a rebuild kit you just about had to go through a dealer at premium prices .Now of days Oregon makes them at much lower prices . Which carb was better has been an ongoing discussion about like the oil wars .I'll remain neutral on this one ,the carb not the oil  :D

Offline HolmenTree

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Re: Trouble with a pair of O38's
« Reply #8 on: October 05, 2017, 03:41:51 pm »
1985 I bought one of the very  earliest 038 Magnums and it came with a Bing carb. Was quite unique to have a German carb on a 038 as my earlier 038AV and 034AV saws had Irish Tilly carbs on them.
But the Bing didn't stay long as Stihl Canada recalled all of them. Then put Tillys and Walbro's back on.
So happened the Bing's castings weren't suited for our Canadian gasoline.
Making a living with a saw since age 16.

Offline Al_Smith

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Re: Trouble with a pair of O38's
« Reply #9 on: October 05, 2017, 07:45:00 pm »
I've got some Tillotsons that are so old they were made in Toledo Ohio .

Offline HolmenTree

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Re: Trouble with a pair of O38's
« Reply #10 on: October 05, 2017, 10:36:28 pm »
I'll take a look through my old stash of carbs.
Making a living with a saw since age 16.

Offline HolmenTree

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Re: Trouble with a pair of O38's
« Reply #11 on: October 05, 2017, 10:37:56 pm »

Yes my old 1970's Jonsereds had Toledo Ohio Tillys ;D
I think our thread OP flew the coup with his 2 posts :D
Making a living with a saw since age 16.

Offline Al_Smith

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Re: Trouble with a pair of O38's
« Reply #12 on: October 06, 2017, 06:15:44 am »
I wonder if in fact since the Bings had some type of plastic material for the fuel shut off lever  if that would cause failures .Never thought of that before this discussion .
Of the three of that model family I have,two Mags and one AV ,two have Tilleys and the newest has a Bing .I've never really had a carb problem on any of them but have rebuilt the internals on all of them,cheap insurance the way I see it .

Offline L34

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Re: Trouble with a pair of O38's
« Reply #13 on: October 06, 2017, 06:59:58 am »
Doing up an 038 mag myself,guess what,cover melted about 15mm back from edge.
Cheers,Chris.

Offline Al_Smith

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Re: Trouble with a pair of O38's
« Reply #14 on: October 06, 2017, 10:11:47 am »
I don't recall if any of mine melted .One Mag had served for over twenty years of hard service for a tree company .I rebuilt that one and it served as a back up saw for 5 more years for  the same company until it was given to me and it still runs good .
That saw BTW literally grenaded a piston,I mean like a bomb went off .Never as much as put a scratch on the Mahle cylinder .New bearings,seals OEM piston .good to go another 20 or more .

Offline JoshNZ

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Re: Trouble with a pair of O38's
« Reply #15 on: October 07, 2017, 09:03:52 am »
I'm still here ahaha, saws had to take a back seat to some other projects we had on, some helping hands organised so it had to happen when it did.

Any sign of why yours melted Chris? The front 15mm of the shroud is the same place both of these are melted. I think the guys here are on the nose, leaking at the manifold or leaking through a split or crack in the muffler itself.

Once again I'd be surprised if the choke mechanism were not working, it behaves as a choke should, chokes it to death if you apply it idling and won't start cold (after day 1) without it. It might take 2-3 pulls cold with the choke on and you get the typical half second fire, choke off, pull, away it goes. Won't get it started any other way. i.e. without choke, or leaving choke on after the half second fire.

But I'll change the air filter and try it - I hope I'm wrong!

It's going to be a PITA of a problem to resolve - being that I have to wait a week+ each time I change something haha.

Offline L34

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Re: Trouble with a pair of O38's
« Reply #16 on: October 07, 2017, 08:59:39 pm »
No idea Josh.Got it as a parts saw but has heaps of compression and no scoring,no vac leaks.So going to do it up.Join in muffler looks good,gasket OK.Going to glue some thin aluminium under new AM cover.
Cheers,Chris.

Offline JoshNZ

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Re: Trouble with a pair of O38's
« Reply #17 on: October 12, 2017, 07:09:39 pm »
I've only just got around to looking at the 038 that is melting its shroud. The gasket behind the muffler looks good, I'm a little suspect of the joint in the muffler though.

Is there any special method that is supposed to be applied when joining the halves? Gasket or gasket sealer etc? Right now the top lip of the front half slips into a slot at the top of the rear half, and then two screws secure it at the bottom, is this correct? The top slot of the rear half is a little uneven, is it supposed to be crimped or filled with something or what?

Thanks again,
Josh

Offline JoshNZ

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Re: Trouble with a pair of O38's
« Reply #18 on: October 12, 2017, 07:22:57 pm »
Here's a photo showing the muffler, obviously it's not joined but you can see the top of the piston side half is looking a little weary.

I wonder if the previous owner though he'd solve the melting issue by bashing the top lip downwards to try and deflect the exhaust in the joint away from the shroud. Might have created a leak. What do you guys think does it need replacing?

Josh


Offline joe_indi

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Re: Trouble with a pair of O38's
« Reply #19 on: October 12, 2017, 09:52:49 pm »
A triangular piece and a holding screw are missing, they keep the exhaust cover tight against the muffler body. Without them there is going to be a leak, upwards