Lawnmower Help

I'm looking for some help from readers. My lawnmower has a problem and I'm not sure what's up. Here's what happened. I was about half-way through mowing the backyard when my mower just idled down. It didn't die, but it suddenly lacked horsepower. There's no problem with the throttle cable or the throttle assembly as near as I can tell.

It quickly dawned on me that I haven't changed the spark plug in at least two seasons, so I did that. Even thought the old one looked bad, the new one didn't solve the problem. My particular Briggs and Stratton engine doesn't have a fuel filter as near as I can tell. I'm wondering if it might be bad gas.

Any suggestions out there?