The foam filter was replaced less than 5 years ago,

That number has no value, it could have been a month or 10 years,
the foam rubber degrades when it degrades and when it degrades
you get a new one.

Later on, the size and shape I liked was no longer available so got
a sheet of the stuff and sewed up my own. The new foam I bought
"bulk" was extra good stuff and never had to replace it, quit
running them before the homemade filters gave up.

...and obviously degraded and softened from fuel vapors.
Downdraft might have had less trouble but this carb is sidedraft.
Considering how little miles I actually put on the ATV, I would
expect a pleated air filter to last more than 10 years and still
not degrade from fuel exposure.

So get the commonly available, cheap foam filter and replace it
every 2 or 3 years. If you are paranoid, wrap it in a nylon
stockong to keep the nasties all in one place.

My son does stuff just like that too, Nick.

Nick, ever ruin a fresh rebuilt 2 stroke instead of putting fresh
gas in it?

Sucking pieces of a foam air filter into an engine because it would
have taken too long to check everything over before starting it,
sounds exactly like a trick my youngest son would pull.

His specialty is to knock loose the steel plate on the bottom of an
air cooled VW engine, then run until it the number-one-rod-bearing
siezes. 4 in a row. One of those just before he toasted it anyway,
limped in leaving an oily line on the ground and he was ready to
take off when I stopped him.

"it's not leaking that bad, I'm not going far, I'll be right back"
"I said, shut the engine off dammit! :/"

Just give him a little more time on your cool foam rubber trick?

Feel picked on, Nick? BTDT too.
How do you think I got to be such a "master' at it?

Alvin in AZ