Can a chainsaw run lean if tachometer shows idle speed and high speed rpms in normal range?

[BHanes]

Hello all,

I have learned so much reading the posts on this website, thanks in advance to all of you who respond. This is my first time posting. Can a chainsaw run lean and score the piston/cylinder if a tachometer (stihl) shows the saw at a “normal” idle speed and high rpm speed without load?

I bought a husqvarna 55 rancher at an auction, compression tested 120. Within a few weeks, middle of this hot summer, the saw started smoking from cylinder. I use stihl 50:1 mix with ethanol free gas. I took off spark plug and muffler and saw scoring of both piston and cylinde. I am fairly new at repairing chainsaws (the ones I use on my farm and for friends) so because the saw seemed to run fine I didn’t replace anything from get go, probably a mistake. I replaced piston and cylinder with meteor brand and replaced carb with new OEM. A friend who is a mechanic of small engines told me to wait on the oil seals because they looked good, and replace them if signs of running lean. I also cleaned muffler screen and gas filter. The saw started right up and I used a stihl tach to get the rpms without load at 12000ish and the idle around 2800. After running it for a few minutes I noticed some smoke coming from near the exhaust port where it seems some excess bar oil was thrown on muffler. This might just be paranoia but the cylinder and spark plug seemed incredibly hot after 3 -4 minutes of idleing and reving. The saw idles well and does not speed up when I turn chainsaw laterally. I looked in spark plug and through muffler and it seems there are a few fine vertical marks all around. I have run saw 2 more times for a few minutes to break in new piston and cylinder (recommendation from mechanic friend). I am nervous about scoring this new piston and cylinder. There is no smoke coming from cylinder, the temp here is in 90’s but it seems saw shouldn’t be that hot after a few minutes. The spark plug is a tannish brown color. I have never done a vacuum/pressure test and need to learn how, but is this normal, can a saw run lean when the tach says it looks good? Should it be this hot this quick? Thanks to all for your help!

 

[CausticUC]

Short answer is yes.

You have three conditions for a basic combustion engine: air, fuel, spark. Of those three, each can cause a lean condition.

Going by your post I'd say your fuel is likely fine, that leaves your air filter and the correct (and correctly gapped) spark plug at the basic diagnostic level.

As for temperature, everything is clean?

 

[stihl86]

Usually, when a saw is running lean it will over rev. Air leaks, is present, will cause a high idle that can’t be reduced.
Or no idle. Don’t fall into the non ethanol hype. Nothing wrong with 10%. Nothing.
If they give a high and low spec, use it.
I always set them a little rich. Nice and fat.
So the spec minus about 2 or 3 hundred rpms
Some try to set them at the perfect 14.7 - 1
Dont be one of them.
You‘re using one of the best oils on the market.
You must consider octane rating. 89 is the norm. 87 could cause problems.
Cheap low octane could wipe out a piston.

 

[buzz sawyer]

There are many, many good threads on carb settings here - have you viewed any?
Anytime you have engine damage, you should do a vac/pressure test to be sure there are no unseen problems.

 

[Robin Wood]

When i was a noobie, i toasted a good running 038 mag chasing too much manufacturers rpm spec with a tach. I realised later down the road listening carefully to the wot no load 4 stroke burbling was more important than chasing numbers

Sent from my INE-LX2r using Tapatalk

 

[BHanes]

Short answer is yes.

You have three conditions for a basic combustion engine: air, fuel, spark. Of those three, each can cause a lean condition.

Going by your post I'd say your fuel is likely fine, that leaves your air filter and the correct (and correctly gapped) spark plug at the basic diagnostic level.

As for temperature, everything is clean?

Brand new spark plug, air filter was cleaned and I cleaned the entire saw with air!

 

[BHanes]

Usually, when a saw is running lean it will over rev. Air leaks, is present, will cause a high idle that can’t be reduced.
Or no idle. Don’t fall into the non ethanol hype. Nothing wrong with 10%. Nothing.
If they give a high and low spec, use it.
I always set them a little rich. Nice and fat.
So the spec minus about 2 or 3 hundred rpms
Some try to set them at the perfect 14.7 - 1
Dont be one of them.
You‘re using one of the best oils on the market.
You must consider octane rating. 89 is the norm. 87 could cause problems.
Cheap low octane could wipe out a piston.

Thanks for the info and wisdom!

 

[BHanes]

Thanks for the info and wisdom!

 

[BHanes]

There are many, many good threads on carb settings here - have you viewed any?
Anytime you have engine damage, you should do a vac/pressure test to be sure there are no unseen problems.

I have read a lot about carb settings. This one has limiter caps, so very little play. Can a new OEM carb cause lean conditions even with limit or cap?

 

[BHanes]

When i was a noobie, i toasted a good running 038 mag chasing too much manufacturers rpm spec with a tach. I realised later down the road listening carefully to the wot no load 4 stroke burbling was more important than chasing numbers

Sent from my INE-LX2r using Tapatalk

Good to know! Now this OEM carb came with limitir caps, should I remove to tune it better by ear?

 

[Robin Wood]

Good to know! Now this OEM carb came with limitir caps, should I remove to tune it better by ear?

Manufacturers recommended rpm are good for brand new saw or fully rebuilt to spec saws, when a saw loses compression from wear and tear or gains from mod work the recommendations set by manufacturers are out of the window.

Lower compression motor might need more fuel aka opening L/H more to compensate the weaker combustion. More fuel accompanied by oil will provide better ring seal and improve power albeit sacrificing some rpms, yes always remove the limiter caps which will give you full range of adjustments

Sent from my INE-LX2r using Tapatalk

 

[BHanes]

Manufacturers recommended rpm are good for brand new saw or fully rebuilt to spec saws, when a saw loses compression from wear and tear or gains from mod work the recommendations set by manufacturers are out of the window.

Lower compression motor might need more fuel aka opening L/H more to compensate the weaker combustion. More fuel accompanied by oil will provide better ring seal and improve power albeit sacrificing some rpms, yes always remove the limiter caps which will give you full range of adjustments

Sent from my INE-LX2r using Tapatalk

Great info! How do I remove caps without damaging carb? Thanks!

 

[rupedoggy]

I use stihl 50:1 mix
the temp here is in 90’s
This one has limiter caps,
I have never done a vacuum/pressure test

 

[Deleted member 117362]

That 120 psi compression was low to start with. Believe that saw is on it's way out to begin with.

 

[buzz sawyer]

Great info! How do I remove caps without damaging carb? Thanks!

Can you send a pic of the caps? Shouldn't be difficult to remove.

 

[farmguywithasaw]

One thing to look at too is your old piston will tell a lot of why the engine failed to begin with. That will lead you to clues as what else might need to be fixed.

 

[Old2stroke]

Throw away the tach and learn to tune the saw by how it sounds in the cut. Manufacturers tach numbers are primarily meant to make the saw meet EPA emission requirements, usually on the lean side of optimum and longevity of the engine is NOT the main concern.

 

[jltrent]

With an air leak they want idle right (erratic) and usually the carb settings want respond when adjusted. Yes you can have proper idle and air/fuel mixture incorrect and toast a piston and possibly a cylinder.

 

[farmguywithasaw]

The op said tannish brown on the main plug which sounds good. The engine will get hot after a few minutes that’s what engines do. Without a temp probe or infrared temp gun you don’t know what is too hot

 

[BHanes]

Update:

Thanks to everyone for their advice and wisdom. I should have said earlier that I do tune my saws by ear but I didn’t with this one because they bought an OEM carb that came with limitir caps. In my naivety, I thought the caps are there for a reason so I shouldn’t take them off to tune it by ear. Thank all of you who set me straight on this faulty logic! The caps just pulled off easily with pliers and I tuned it to run richer, which I feel good about. I really appreciate all your help and look forward to learning more about saws on this forum!
Ben