Question on Stihl 025 Tuning

[2lumbarleft]

Gentlemen, I have a fair amount of experience tuning various chainsaw makes and models, but I don't do much work on Stihl chainsaws. I have recently received an older Stihl 025 that was rebuilt by a self reputed chainsaw expert. He had installed a new piston, rings, carburetor, fuel lines - the works. When I received it, it was running terrible. I started basic trouble shooting. Spark plug was pig rich. I checked the initial carburetor settings and they were way off - ultra rich. I started at the normal beginning settings, 1 turn out on the low speed jet and 3/4 turns out on the high speed jet. I cranked up the throttle screw and went for restart. The saw came to life and was running better, but I noticed the jet adjustments were very sensitive. I used the official Stihl method of setting the carburetor. I set the high speed rpm to 12,500. Here is where I departed the official tuning method. I have the low speed jet so lean that 1/8 of a turn clockwise will kill the saw, yet there is plenty of fuel for snappy transition 1/8th turn back ccw. The saw definitely will run better if I lean up the high speed jet, but just 1/8 of a turn will jump my rpm to 13,500 which I know can't be good for a class C homeowner saw. I am running 45:1 Amsoil Saber Pro with premium non-ethanol gas. My problem is that I am still blowing snot out of the muffler. I know in my guts that it should be leaner, but I am afraid of the rpm increase. The saw runs good, but should I be concerned about the appearance of too much oil ejection from the muffler? It is always wet and there is a small build up of black oil gas mix below the muffler. Has anybody else experienced this, or is this normal for older Stihl chainsaws? I will say that with 195psi compression this thing has guts, but I have already ripped off the recoil starter handle. Replaced handle and the rope broke one inch down. Replaced the handle and broke the rope 10" down. Installed a new rope and no problems so far, but my rotator cuff is talking to me - ha! Sure would not want to start this saw up in a tree. I am thinking this thing is muffler modded, but I don't want to run this saw at pro saw speeds. The saw was not designed for those kind of stresses. Thoughts?

 

[HarleyT]

Sounds like a crappy chinese carb problem

 

[2lumbarleft]

Sounds like a crappy chinese carb problem

I am thinking you might have nailed it right off the bat. I was afraid of that, but wanted to see if anybody else had similar experiences. Seriously, the saw runs great, cuts good, but I know it is too rich (especially with black plug color and snot out the muffler), and the adjustment values don't make sense. It must be the carburetor. I doubt running the 45:1 fuel mix would cause that kind of a problem, and the saw just does not respond to normal Stihl tuning techniques, or any others for that matter.

 

[ericm979]

My one owner unmodified 025 does not drip spooge out the muffler.
High compression and a hard pulling starter is not unusual on these saws. It's ok if you're starting it once and running it for a while but if you stop the saw often to move material out of the way it can get difficult.

 

[2lumbarleft]

My one owner unmodified 025 does not drip spooge out the muffler.
High compression and a hard pulling starter is not unusual on these saws. It's ok if you're starting it once and running it for a while but if you stop the saw often to move material out of the way it can get difficult.

I have run plenty of initial compression check readings on 025 and MS 250 chainsaws. I usually get about 155 psi (tops) on a fresh engine. 195 psi is ridiculous. I would guess a base gasket has been eliminated to reduce the squish. You are right in that even regular stock Stihl MS-250 and 025 saws pull hard compared to others. I am convinced that the problem lies in the low speed jet. With the slightest adjustment, the saw goes from too lean, to too rich. It flows enough with a minor adjustment to effect top end rpm by 500 and significantly reduce rpm for the idle. I am going to keep an eye on the piston for carbon build up. That should confirm my diagnosis. I will probably end up replacing the carburetor with a factory OEM product. Either way, I doubt if the crank seals will hold up long with 195 psi readings.

 

[hotshot]

No base gasket is used on a clamshell type engine...you’re probably spot on about the excessive carbon buildup.

If the Welch plug in the new carb is leaking by, adjusting the low speed mixture screw will be moot. Even though it is new, you need to remove & pressure test the carb.

I’m not sure about your last comment about the crank seals, are you suggesting that the high compression will eventually destroy the bearings, and the seals will follow?

 

[2lumbarleft]

Actually regarding the last remark, that is exactly what I was suggesting. This is not a vertically split magnesium crankcase. That is allot of stress to place upon a plastic modular constructed saw. If it was of no consequence, manufacturers would have adopted those cheaper changes long ago. I am not saying it will fail soon. I am just positing that it will not deliver the same life span as in a superiorly constructed saw chassis. I will leave that debate for superior minds than my own.

 

[HarleyT]

High compression numbers are likely due to the flooding carb.
A new OEM Stihl carb is only @ $24

 

[HarleyT]

The superior minds here were banned @ 4 years ago, or so....

 

[2lumbarleft]

High compression numbers are likely due to the flooding carb.
A new OEM Stihl carb is only @ $24

I am going to order a new OEM Stihl Carb. Thanks for the advice. Makes sense to me. Too cheap not to try.

 

[SteveSr]

This is a clam-shell saw. There is NO base gasket to delete. So to get that much compression you likely have a wrong piston or the combustion chamber is chock full of carbon from being run too rich.

 

[2lumbarleft]

This is a clam-shell saw. There is NO base gasket to delete. So to get that much compression you likely have a wrong piston or the combustion chamber is chock full of carbon from being run too rich.

 

[2lumbarleft]

Thanks for adding more understanding to this issue. I haven't removed the muffler yet. I have however, removed the spark plug and viewed the piston. There is a little carbon, but the cylinder looks fairly clean. If the the wrong piston was installed, I am surprised the saw runs so well. I am not wrong on the compression reading. I used several gauges and all were close. 195 psi was lowest reading. It is a bear to start.

 

[ericm979]

I used a borescope to look inside my 025 and examined the side and top of the piston through the exhaust port. Not a lot of carbon. But when I do the calculations it does not take much to increase the compression significantly.

 

[2lumbarleft]

I used a borescope to look inside my 025 and examined the side and top of the piston through the exhaust port. Not a lot of carbon. But when I do the calculations it does not take much to increase the compression significantly.

As an experiment, I think I am going to switch over to 50:1 Shindaiwa/Echo Red Armor for a while. This morning I am going to pull the muffler and inspect that little oil plug that is supposed to be under the muffler on the left side as you face the cylinder. I understand from videos that a missing plug can make a mess also, but still doesn't explain the ridiculous compression. I just can't believe that the builder of this saw was stupid enough to use the wrong piston, and as another contributor pointed out, there is no base gasket to eliminate in a modular saw (I was wrong on that). So what remains has got to be carbon build up, even if it doesn't look significant. I still think the advice from others to replace the carburetor with an OEM carburetor is the way to go. Even if that doesn't work, I will have a spare on hand for the future.

 

[2lumbarleft]

As an experiment, I think I am going to switch over to 50:1 Shindaiwa/Echo Red Armor for a while. This morning I am going to pull the muffler and inspect that little oil plug that is supposed to be under the muffler on the left side as you face the cylinder. I understand from videos that a missing plug can make a mess also, but still doesn't explain the ridiculous compression. I just can't believe that the builder of this saw was stupid enough to use the wrong piston, and as another contributor pointed out, there is no base gasket to eliminate in a modular saw (I was wrong on that). So what remains has got to be carbon build up, even if it doesn't look significant. I still think the advice from others to replace the carburetor with an OEM carburetor is the way to go. Even if that doesn't work, I will have a spare on hand for the future.

 

[HarleyT]

Drain the fuel out and take the plug out, pull the rope a bunch, and leave in the sun for a few hours, make sure that it is totally dry. Then re-check the compression. If still high, then I would vote for the carbon.

 

[2lumbarleft]

Drain the fuel out and take the plug out, pull the rope a bunch, and leave in the sun for a few hours, make sure that it is totally dry. Then re-check the compression. If still high, then I would vote for the carbon.

 

[2lumbarleft]

This morning I removed the muffler. It was loaded with oily tar that could be wiped out with a paper towel. I was pleased to see the muffler was of the older design that could be disassembled for cleaning. I could see oily residue on the exhaust exit port from the cylinder where it meets the muffler. The spark plug was black! Here is the head scratcher. The cylinder was pristine and the piston looked like it had been freshly installed. There wasn't a speck of carbon! I believe that this a testament to the use of Amsoil Saber Pro 100 mixed 45:1 with 1 ounce of Sea Foam per gallon of non-ethanol premium fuel! Obviously the saw was running way too rich, but it was running fantastic! I spun the piston with the recoil starter many, many times. I then let the saw set out in the sun and bake for several hours. I put everything back together and took a compression reading. 195 PSI - Ugh! I have no idea what the rebuilder of this saw did, and I have lost his name and telephone number, because this saw has just been sitting for the last two years in storage. The cylinder shows signs of having been rebored, probably from a previous scored cylinder episode, and the piston rings look absolutely brand new and fresh without the slightest sign of scratches. I will put a new carburetor on this saw, but I thought I would share the head scratching with others.

 

[2lumbarleft]

This morning I removed the muffler. It was loaded with oily tar that could be wiped out with a paper towel. I was pleased to see the muffler was of the older design that could be disassembled for cleaning. I could see oily residue on the exhaust exit port from the cylinder where it meets the muffler. The spark plug was black! Here is the head scratcher. The cylinder was pristine and the piston looked like it had been freshly installed. There wasn't a speck of carbon! I believe that this a testament to the use of Amsoil Saber Pro 100 mixed 45:1 with 1 ounce of Sea Foam per gallon of non-ethanol premium fuel! Obviously the saw was running way too rich, but it was running fantastic! I spun the piston with the recoil starter many, many times. I then let the saw set out in the sun and bake for several hours. I put everything back together and took a compression reading. 195 PSI - Ugh! I have no idea what the rebuilder of this saw did, and I have lost his name and telephone number, because this saw has just been sitting for the last two years in storage. The cylinder shows signs of having been rebored, probably from a previous scored cylinder episode, and the piston rings look absolutely brand new and fresh without the slightest sign of scratches. I will put a new carburetor on this saw, but I thought I would share the head scratching with others.