Thanks for the responses.
1. "Take the saw to a good mechanic." That is great advice, of course. It was my first thought when I started having trouble. The problem is that there are no good mechanics within at least 20 miles. I tried. I don't want to bore anyone with the details, but I really had problems when I tried this, and I only used Echo-authorized businesses. Imagine having your saw held hostage for almost three months by incompetent repair joints. It happened to me. I can't figure out why they're so bad or why they have multiple-week backlogs. They also charge over $100 per hour, which is beyond insane. People coming out of law school can't touch that.
2. "Use the warranty." Echo's warranty specifically excluded the carburetor! That was brilliant of them.
3. "Tune it." I wish I could. Can't tune it until it runs steadily. I love watching videos about saw problems. "First, start the saw and let it run a while." If people could do that, would they be watching videos? The tuning directions are simple. Making the saw run is the hard part.
4. "The carb is messed up." Third carb. Right now I have a carb for a 620, bought new. I know how to take a carb apart, and I have a sonic cleaner, but it generally doesn't help with my small engines. I clean with hot gasoline in the machine. You would think that would break anything loose.
As long as I'm complaining, I have to say that I don't like Walbro. When I got this saw, I could buy a Chinese carb for just about any other small engine for $12. I'm just now seeing Walbro clones, and I got this saw in 2017.
Think about this: if I had a carb problem, I had a choice between fixing it myself, spending $125 per hour for a doofus to keep the saw for over a month, spending $100 or more for a new Walbro, or trying to make a Chinese carb work. The economics are unbelievable for a $400 saw.
I think I have problems largely because I'm in Florida and I don't use the saw very often. Being in Florida, I never have reason to clean it up and store it at the end of a season. There is no season, and I never know when I'll need it. Because my wood-clearing problems are separated by long down periods, the saw has a lot of time to sit in a hot, humid shop and gum up. I've tried Trufuel as well as running the saw dry, but I still have trouble.
The reason I think another saw might be better is that I have other small engines that never give me trouble. If Echo can make a blower that will run after a year and a half of sitting with fuel in it, somebody must be making a saw that won't gum up and die in 6 weeks after being run dry.
I use Biobor EB and Red Armor in everything, and I use zero-corn gas. I admit, I have not been great about looking after things. This year, I started up my Echo blower, and the gas was from April of 2023. Fired right up and ran like a champ.
The Makita is infinitely superior to the Echo in every way except for run time and bar length. I can do a 90-minute job with 4 batteries, and then I'm done for the day. I wish I could connect it to a car battery! I'd never use anything else.