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I’m not a pro, but I am the go-to guy for some of my friends when they have issues. Recently one brought me his Stihl chainsaw which would not start. It was less than a year old, but it was too much hassle for him to find the proof of purchase and bring it to a dealer for warranty work. He also brought me his neighbor’s identical saw, the one that inspired his own purchase. Maybe a year older, it wouldn’t start either. Of course, by this time they were both badly flooded from repeated attempts to start them. That was easy enough to remedy by inverting each saw and pulling the starter rope a dozen times with the spark plug removed. Then it gets interesting. Both plugs were gapped, probably by Stihl, smaller than the minimum acceptable, maybe 015 rather than between 020 and 050 inches. You can’t really expect a chintzy little spark to do its job very well, so I re-gapped them close to the middle of the proper range. The older saw had one additional problem, the spark arrestor screen (or whatever it is) in the muffler was totally clogged with soot, so the back pressure must have been terrible (see photo). I figured both saws were better off without them and proceeded accordingly. They're fine now.

A torch like this works well for cleaning those spark arrestor screens

Depends where you use them
Down here if you get caught using one without the arrestors in a national park or state forest you do time.
No if or buts, starts at 12 months inside or 2 years full time community service .
Most never use a saw at night so you don't see just how many glowing embers come shooting out
Add sawdust to that and a bit of a breeze & you have a forest fire.
Currently a "chain saw" act is on the books which will allow the police , park rangers , council inspectors to confiscate any saw at any time if found to have the arrestor removed.
The sticking point is what to do with them.
Greenies want them scrapped , conservatives want them repaired & sold at auction socialists want them repaired & returned to the owner after they pay the fine.

As for the two saws you just got running, you have not fixed either of them and the carbon that was building up on the spark arrestor is also building up on the exhaust port.
The factories set the saws to run rich from new to prevent lean burn seizure.
Usually they have to be leaned off a little to run clean

Alright, both screens are now clean and back in their respective machines. Rest easy.

Screens clog because:-
the main jet is set too rich
The fuel mix has too much oil
The owners labour the engine with a dull blade
In any combination of the 3

The tell tale that it is clogging is the saw will sit & idle all day but bogs down under full throttle
And the time for bogging down gets shorter & shorter it it won't take throttle any more .

A dirty air filter is a cause as well.