Folks, I have an older Craftsman lawn mower that I can't get to start.
I do get a good spark, so I'm assuming that it means a clogged
carburetor. Is there anything that I can pour into the tank and allow
to sit there that might help dissolve any old varnish? Thanks in
advance for your suggestions!

You could get an electric, learn to do the "cord dance," and never worry
about starting the mower again. With an electric, you just turn it on. If
it doesn't start, buy a new one, they're only $150 or so. End of problem.