How do I know when to rebuild my Stihl?

[Moto Vita]

I have two 036 Pros that I bought used, one is much more used than the other, it works ok but it takes a few more pulls to start and doesn't have quite the power of the better one. Now that I'm shut down for the summer I put a compression gauge on the weaker saw, I could only get 140psig. My gauge isn't a high quality Snap On or anything so I expect there could be a plus or minus factor.
I got out the service manual and found that Stihl doesn't list a pressure range for the top end, only pressure and vacuum tests for the bottom end. They also don't give any specs for piston clearance. or diameter or ring gap, am I missing something?
So if I take it apart how do I decide what to replace? And what is likely to happen if I run it too long? I'm a motorcycle guy and in my old two strokes if I ran a piston too long after it started slapping a skirt could break and make a mess, does that happen with modern saws or does performance decline to a point that a rebuild is necessary before anything usually comes apart?

 

[ChoppyChoppy]

140 is fine. I run an 036 that has 95psi compression. It's tired, but still runs ok.

 

[HarleyT]

Just for giggles, swap the carbs and see if they runs any different.

 

[northmanlogging]

Run em till they don't, then rebuild em.

Or pull the muffler and have a peek at the piston, if its all scored and gouged, time for a rebuild.

 

[HarleyT]

Runs.....

I was going to edit that, but I like it better this way.

 

[David Guptill Jr]

Pull the Muffler, and look to see how much (if any)scoring there is.. the muffler side runs the hottest, and is more likely to score, Not always, but most likely.
That being said, if there is low compression and you dont see scoring, it could be on thw intake, some will run without em run with old, holy air filters, or none.. then the intake side can be scored..if the saw is Old.. and sitting for a long time, the rings can get locked up, this can be remedied with Marvel Mystery Oil and leaving sit overnight, and I'm sure there's other things that work very well.

This are just a few of the problems I have seen.. to many to list all..

 

[rynosawr]

140psi isn't a bad reading at all, not even depending on what elevation you are at.

"Most" saws won't run strong when the compression gets below 110psi, and some won't run at all if below 100psi.

If the piston isn't scored on either side, and there isn't much or any carbon on the rings, the rings are just likely a little worn.

If the piston and cylinder are good, new rings would Give another 15-20psi I bet.

Probably wouldn't make much difference.

I would check the ignition module/coil for cleanliness and gap distance and maybe look at a carb kit as well

 

[Moto Vita]

Thanks guys, the saw runs well enough that I'll just clean it and leave well enough alone. I was mainly worried about piston/bore wear that would lead to a breakdown, It doesn't sound likes that's a big concern. The piston looks fine through the exhaust port.

 

[JonCraig]

What does the same gauge read on the "good" saw? That would give you an apples to apples comparison.

I freaked out when I bought a (cheapo) gauge that told me my 460 had 120psi, until I put it on my 391 that had probably had only 2 tanks through it and it also read 120 psi...

 

[northmanlogging]

all this head ache could just be something as simple as a slighty different tune on the carb side of things, a tiny turn one way or the other drastically effects power.

Might be worth yer time to take it to a good dealer and have em tweak the carbs a little, if anything then it won't be running lean.

 

[Westboastfaller]

When Im digging it out from under a tree has generally been a good indicator in the past.

 

[HarleyT]

Swap the carbs, it would take 10 minutes to do, and cost you nothing, and point you in the right direction.

 

[HarleyT]

If you swap them, and then the other saw performs in that way, then you will KNOW that it is the carb, and not the piston/cyl., or whatever. Then you could focus on that carb, and learn a great deal.

If the saw performs the same, well, then you will still learn a lot as well, eh?

Probably needs a fresh carb kit, as a saw doesn't perform well with old, stiff diaphrams.

 

[Moto Vita]

The rougher saw doesn't run bad, just not as strong as the nicer saw, it was used commercially and it's overall condition suggests it has seen lots of long hard hours. My main concern was that I didn't want to run it until a piston skirt broke or some other catastrophic failure occurred, you guys have assuaged those fears for me.
I decided to order a carb kit for it since cold starts take more pulls than my other saws, warm starts are normal. I'm guessing that fuel could be leaking back out of the carb past a stiff old diaphragm.
An interesting anomaly of this saw is that it's difficult to get a good rpm reading on my wireless digital tach. The tach display will frequently go to all 8s and then reset, it can take multiple resets to get a good reading, none of my other saws, all Stihl, do that. Anybody have any comments on that?

 

[Wood Doctor]

If you swap them, and then the other saw performs in that way, then you will KNOW that it is the carb, and not the piston/cyl., or whatever. Then you could focus on that carb, and learn a great deal.

If the saw performs the same, well, then you will still learn a lot as well, eh? Probably needs a fresh carb kit, as a saw doesn't perform well with old, stiff diaphrams.

I did exactly this with a pair of 028 Supers. Before I swapped, one would run OK and the other would not start and run at all. After the swap, they both started and ran perfectly. Needless to say, I was pleasantly surprised. It's the nature of the business. Some things cannot be explained. That was two years ago. Both saws still run fine today.

 

[Okie]

don't forget the little measley spark plug.

Don't trust your comp gauge on a chainsaw unless you are familiar with your gauge and know that it's capable of reading a small CC engine correctly. and it's also indicating enough compression to operate ok.


Swap the carbs if a spark plug don't fix it. (most likely the carb)

Keep a heads up and don't operate it leaned out.

Some of them Stihl OEM carbs are about same price as the kit. (unless you just like to do carbs and then sometimes a kit won't fix a carb that is gummed up under a welch plug)

Also run non-ethanol regular gas and a Stihl oil mix for longer carb and saw life.

If none of these fix it and it will start, just run it until it quits and then repair or use it as spare parts for your good one.