I have this small tube-type tire on a wagon. It went flat. I took the
tube out & checked it under water - no leaks. I put a new stem in & put
the tire back on the wagon. It went flat!

I gave up & tried something that I saw on YouTube - foam. I drilled
some holes in a sidewall and squirted in some Great Stuff. I left it to
harden and came back to this:

http://imgur.com/a/YcGpa

Not a problem, but definitely amusing.

Bob