lawnmower hesitating, using too much fuel
I'm the proud owner of a 13-year-old MTD push mower, with a 4HP Briggs&Stratton engine. Has worked like a charm throughout its life, until this spring it developed some troubles. While I was mowing one day in rainy weather (don't ask), the mower started hesitating a lot, going to low rpms regularly. This problem stayed, even after changing oil, inspecting the spark plug, starting with a fresh batch of fuel (it contains 10% ethanol around here, but that's not been a problem before). Also, it uses about 50% more fuel than it did previously, which has me concerned that I'm putting the additional fuel up into the air as pollution, by running too fuel-rich. The hesitations occur especially when the mower is changing directions (backward to forward, around a turn, etc).
I took the mower apart once to see if I could find a carburetor adjustment of some kind, but all I found was a little vane at the top fan, which does not appear to be adjustable.
Any suggestions for an approach to get the mower back to prime working order? I'd like to get another 13 years out of this puppy (or at least until I turn enough lawn into garden that I can cut all of it on a single charge of a battery electric powered mower, which I've been eyeing).