Looking for 1970s Children's/YA Mystery

[klwill1192]

I'm trying to track down a book I read as a kid. It was a 1970s era YA mystery. Here are the few details I remember:
The main character had a friend who was a genius inventor. One of his inventions was a retractable staircase.
The main character had a newspaper route and was in a contest to sell subscriptions.
One of the mysteries involved what turned out to be a (friendly) family of circus performers.
At the end, the circus family purchased a number of newspaper subscriptions that resulted in the main character winning (I think) the subscription contest.

I really don't remember much more about the book, but I'm hoping these details will ring a bell with somebody.

Thank you.

Kevin