You have a small diaphram on the buttom of that carb.

There could be a pinhole leak in it. I would say
take that carb completely apart and rebuilt it
with a rebuilt kit (I think you said it was a sears unit so
its actually pretty easy to get)

While its apart, I would use gumout and spray it in all the
passages. Make sure it comes out of everywhere. Use thin wire
to clean everything out.

Tom