Carbon build up on exhaust side of piston. Bad?

[Marshy]

... As far as getting too dependent on the use of a compression gauge, I don't rely on them too much..

carbon buildup gives you false compression readings, I only use a compression gauge
when I am selling a somewhat complete chainsaw carcass on Ebay.

I have seen saws with horribly disfigured pistons give high compression readings.

Is that a joke because it made me lol. What do you use to monitor/determine engine health, engine dyno... hand and ear dyno?

I'm doubtful you could have enough carbon to give a false reading of more than a couple of points. It won't mask a change of 20 psi. Compression gauge is only as useful as the person holding it IMO.

 

[HarleyT]

Glad to help you "lol"........

Not sure how to respond to that. I have 3 tachs, and many torque wrenchs that I rarely use as well.

If you need some gizmo to tell you when you have a saw with poor compression, then that is your cross to bear. The pull rope usually suffices for me.

 

[ANewSawyer]

Is that a joke because it made me lol. What do you use to monitor/determine engine health, engine dyno... hand and ear dyno?

I'm doubtful you could have enough carbon to give a false reading of more than a couple of points. It won't mask a change of 20 psi. Compression gauge is only as useful as the person holding it IMO.

I don't follow you. What reading are you talking about?

 

[Marshy]

I don't follow you. What reading are you talking about?

Compression reading. With enough carbon built up in the combustion chamber the compression number will raise and show a "false" higher compression. However, what I'm saying is the amount of carbon that would be required to falsely raise the compression more than a few points (psi) would have to be very significant. At that point you'll likely start to get some scoring on the piston and overall compression will start degrading noticeably and lack performance becomes obvious.
If you have a new saw and test the compression when new and compare it after say 150 hours of use and it's dropped 20 point it's probably time to take the muffler off and look at the piston for scoring. The compression reading is the most common quantitative measure of engine health, everything else is subjective.

 

[HarleyT]

A small amount of carbon buildup will change the compression ratio quickly. As does a thinner cylinder gasket, etc...

 

[Marshy]

A small amount of carbon buildup will change the compression ratio quickly. As does a thinner cylinder gasket, etc...

So how long do you recon it takes to build up enough carbon to increase its compression reading above what the saw comes with brand new? Might be hard to tell if you dont use a compression tester eh?

 

[HarleyT]

I don't have a problem with you using a compression gauge, so why are you so worked up?

I don't need a compression gauge to figure out if there is something wrong with a chainsaw.

Imagine a .5mm layer of carbon atop the piston, wouldn't that be the same as going with the thinner cylinder gasket? One of those gaskets are denoted as
"low compression" in the Stihl parts list.

I work on saws, and in the past have worked on saws with this symptom, and have found that the excessive carbon causes the cord to yank out of you hand, as does a
little jot of oil in the cylinder does.

We are not really disagreeing with much from what I see, other than my not finding a need for as compression gauge. I find that pulling the rope is enough info for
me. That is all...