Question about Stihl Chainsaw 42mm Piston Rings 025 O25 Chain Saw

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Have stihl O25 chainsaw, starts, runs about 5 seccinds and dies, start and it repeats. have changed spark plug 2 times ,changed to new gas, does not help

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Hey have the same o25 stihl,great saw when their working right.have had to replace fuel line to carb 5x over 7yrs.hairline crrack and it will **** air and conk out in 2-5 seconds.also there is a pulse line to carb which can also crack.if you have good spark at plug its likely a fuel delivery problem. collin

Posted on Jan 15, 2011

  • mixmasterc Jan 15, 2011

    since when is **** a starable word?

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DISCLAIMER. CHAINSAWS ARE DANGERIOUS. THEY WILL ALWAYS BE DANGERIOUS. THEY ARE MEANT TO BE DANGERIOUS. YOU SHOULD TAKE YOUR CHAINSAW TO A QUALIFIED TECHNICIAN TO BE SERVICED/REPAIRED. HOSPITALS ARE FULL OF PEOPLE WHO THOUGHT THEY KNEW WHAT THEY WERE DOING...!
First thing about Stihl 021, 023, 025 to look for if you have one that won't start, starts, and stops, runs ,but erratically, Ticks-over sometimes, then races, or stalls, Revs up, then races and dies.
You have an air leak somewhere. It is affecting the mixture.
You have done a carb kit, serviced the saw, and even maybe changed the main oil seals... It has compression... It's got fuel, and a spark... But it still runs like somthing that fell out of a dogs backside..!
Check the Impulse Hose.
It is totally hidden on these saws.
If you have had to replace your fuel delivery pipe. The chances are that the Impulse Hose is also shot.Even if it's not perished. They often stretch, and leak at, or fall off of the handle housing end. Poor non-sealing impulse hose gives a wildly varying fuel supply to the saw. As the received pressure impulse controls the movement of the Diaphragm in the carb. Thus controlling the fuel supply.

This hose is hidden like you won't believe. The "spigot on the Handle Housing has no barb on it. It's just a straight bit of orange plastic. The Impulse hose slides over it. There is no clip to hold it in place.
If you peer into the little gap between the top of the fuel tank. And the underside of the air filter housing. You might see the pipe.Look from the chain side of the saw.
Best to take off the Starter (recoil) Cover. This is when you realise that you haven't cleaned your chainsaw properly ,for a very long time... !
Look at the back of the cylinder. Underneath the inlet manifold. You should see a black corrugated hose .It fits over a brass ferrule. Sticking out of the upper crankcase/cylinder casting. It points towards back of saw
Anyway. Follow this hose. It exits through the white plastic "Baffle Plate" (item 20 on the drawing) And snakes up to fit on to the spigot on the Handle Housing item 1 , 2 (on drawing) Use a hooked piece of wire to GENTLY tug the hose. Try to find where the other end goes to. As I say there is a plain Plastic spigot, pointing vertically downwards, in the Handle housing. It is hidden up and out of sight. But it is there. Honest. Unless it is broken off! ..Not likely... It doesn't need to be broken ,to be disconnected though.
To change it. You will probably need to remove the carb, and the housing the hose joins onto.
Clean it all up.
I skip over describing how to remove these cases. Because 1/ without a machine here to actually work on. My description would be less than complete/reliable. 2/ If you don't know how to do this basic disassembly/reassembly stuff anyway. I don't think you should be messing with machinery in general ,and chainsaws especially. I wouldn't want your amputated limb on my conscience. Or your lawyer on my back... !
Refer to the drawing (at end of post)
Handle housing is item 1,2 (different versions)
Impulse hose is Item 19
Look at item 20 Baffle plate.

Top hole is for spark plug
Bottom hole is for inlet manifold
Just underneath it is a much smaller hole. The impulse hose passes through this to connect on to the ferrule on the crankcases.
Fit your new hose to the orange plastic Handle Housing first.
When refitting the Handle Housing. Put the Impulse hose through the little hole that's made for it in the white plastic baffle plate. ..Towards The Engine End brass ferrule it fits over. ..and fit it to the ferrule... You can get to this end easier than the handle housing end, when the saw is assembled.
I find that it is slightly less hard to fit the hose this way. Than trying to connect from the engine, end to the housing end. Unless you have unnaturally long /thin extra jointed fingers
But it remains a fiddly frustrating task. Just make sure that the impulse hose is well fitted on the handle housing end. I have used one of the metal clips, used to hold early type fuel filters on to hoses ,to secure the impulse hose. But you have to be careful. To close it up nice and round. Not to tight so as to crush and damage the hose. Stihl would not agree with doing that... Your choice...
When You have it all back together.
The baseline setting for both your carb fuelling screws is
1 turn out, from LIGHTLY seated.
Turning in (clockwise) reduces the fuel supply...lean mixture
Turning out (anticlockwise) increases the fuel supply...rich mixture
If you have a rev counter. MAX SPEED, with chain and bar fitted, is 12500rpm, regardless of bar length, or chain size.

If you don't have a rev counter...best advice...don't turn the Hi speed screw to less than 1 turn out. More revs/less power comes from a lean mixture. Overheating/seizures come from an overlean mixture.
Tick over should be 2800 rpm...good luck with that...!
I have been working small engines for 30 years.
I have been working for Stihl for 4 years.
I really dislike this particular series of Stihl chainsaws. I have never had a good one come through my workshop. They are always worn out too much to fix properly/economically. I shouldn't really blame the saw. I should blame the owner(s) that still expect a 20 year old saw, that has never been maintained properly, to be repaired like new ,for less than a couple of hundred bucks.
I have had half a dozen of them through the workshop in the past year. They are all worn out to the point that even with, Carb kit, impulse hose, and new main bearing seals, filters etc,etc. I can never get them to run a good steady tick over. They will run at3000rpm + a bit.Or drop down around 2100 rpm . Still useable, as a chainsaw. But not up to the standard I set myself for repairs. They are just too worn to run well. A new carb might help.But no one wants to pay for one! I don't have one to try, to test out my theory.
I would rather use either a standard 009L (40 cc) or better a 009L fitted with an 012 cylinder/piston (45cc) Small, light, gutsy little saws.I still get these through the workshop. Most of them come up well with a standard service. Even though they are older than any 021,023,025.
Refer to the drawing
Handle housing is item 1,2
Impulse hose is Item 19
Look at item 20 Baffle plate. Top hole is for spark plug
Bottom hole is for inlet manifold (item 18)
Just underneath it is a much smaller hole. The impulse hose passes through this to connect on to the ferrule on the crankcases.
I hope this is of some help to all you 021,023,025 owners out there.
have stihl O25 chainsaw, starts, runs about 5 - 8b8wbvw+bm8vwaaaaasuvork5cyii=

Posted on Dec 01, 2014

  • Hit it again
    Hit it again Dec 01, 2014

    Sorry cant get my drawing into an acceptable format to load onto Fixya.You will have to get by without it.

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I can't help with a solution but I have the same problem...new plug, cleaned air filter, fuel filter, exhaust, set carb ajustments to basic settings as per handbook....makes no difference. Am about to strip the whole ****** down but if anyone had any advise orv suggestion it would be much appreciated

Posted on Jan 14, 2011

  • Sunwhite Jan 26, 2011

    Right on the button :-)... Turned out to be the fuel line on my 025. Grins maybe ***** was a typo on the word **** ???

  • Sunwhite Jan 26, 2011

    Nope...just doesnt like the word s-u-c-k.......

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