Mower runs, but has no 'ummph'?

I was merrily mowing my front yard when the mower suddenly lost it's power. It still ran, but putt putted, and if I went in to grass it couldn't handle it. It's a basic Murray push mower w/ a 4 hp Briggs & Stratton. Been fine for years. I cleaned the air filter and changed the plug, same problem. Interestingly, if I tilt it back, it picks up momentum, but slows down again when I put it back down. I took the blade off earlier today to sharpen it. I'm wondering if maybe some garbage got in the fuel line. Any insights would be appreciated.
Thanks