The root problem here is the fact that Echo is hell bent on pleasing the EPA and turns their power equipment loose pretty lean right out of the box. Even so in the hands of the average homeowner or non-commercial use they fair pretty well.
IF a few simple steps are taken BEFORE you place the saw (or other power equipment) in service it will not have a P/C issue.
I know this because I work on power equipment here, and see "smoked" P/C from all brands, and most of the time it's simply from the operator running them in a poorly tuned condition where they aren't getting the fuel they need to keep EGT's down and adequate lubrication to the internal components.
There are always a few that come in where the owner should have hired the work done and kept his day job. This includes picking up the WRONG fuel can without any oil mixed in it, or fuel so old it I can pour it in the driveway outside the shop and have trouble lighting it with a torch! Yes, just had this happen last week when I guy brought in a Honda 2500 watt generator that would not run. Absolutely NOTHING wrong with it aside the fact that the fuel wouldn't burn, but he thought, like most other homeowners than the Stabil he put in the fuel back in 2005 would preserve the fuel no matter how long it sat till the next power outage........lovely!
Anyhow, these Forums exist to educate folks, so here goes. When you buy an Echo saw (or other brand that has a carb with mixture screws/limiter caps) remove them and modify them so you can custom tune the saw if/as needed. This is where the Echo haters chime in and scream you'll void the Warranty, well, IF you follow these directions you aren't going to need the Warranty and done correctly no one will be able to notice that you modified them anyhow. (pics below)
The manufacturer's do try to set them up so they don't have issues, but there are WAY too many things beyond their control, including fuel type, octane, amount of ethanol, quality (yes some fuel isn't as good as others), how old it is, quality and amount of oil mix being added, altitude, DA (Density Altitude)/air quality, failure to clean the air intake system/air filter, etc, etc, etc, etc.
So IF you want your little Echo to always be at it's best, learn how to remove the limiter caps, modify them, put them back in place, and make correct carb settings based on the fuel you are using and parameters the saw will be used in.
IF you want to be with the masses and simply buy a saw that gets casual use, fuel that's sat in a gas can for months or even years, and don't really care to have your saw always be at it's best and last a lifetime, then stay off the Forums and sit in your recliner and have the wife fetch you beers instead.
To this day it AMAZES me how well some of this equipment lasts with the abuse and neglect that it gets from the owner/operator. To make that deal even worse those folks come on here whining like little baby's AFTER they have smoked the P/C......and NEVER put Stabil in saw gas, just use it up within a couple of months, then throw it out before dumping it into any of your power equipment, ESPECIALLY if it wasn't in a sealed container as ethanol soaks up water 1 to 1 and forms "apple jelly" in the fuel can, saws fuel tank, and ends up plugging up the tiny passages in the carb (Mountain Lake refers to this above) leaning out the A/F ratio even further. As a side note here I absolutely LOVE ethanol, it keeps us BURIED in work in the shop with no end in sight.
In closing and FWIW, I can purchase an Echo CS-590 for a little over $300, have it shipped here, quickly remove the limiter caps/custom tune, give it to a company who removes trees for a living, and their ground crew will run the dog-living-piss out of it every single day, and run into them many months or even years later and they tell me the saw still runs flawlessly and has needed nothing but a few dozens chains replaced and a bar or two. IF I were to simply gas the same saw up, fire it up for a quick test run, hand it over to the same company.......it would be back in here in less than a month with a "smoked" PC, guaranteed!........FWIW........Cliff