Briggs-Stratton Quattro

Seven-yr-old MTD 22" rear-bagger. 4 hp Briggs-Stratton Quattro,
well maintained, doesn't smoke, oil level fine, carb is
clean in/out, air filter has been cleaned.

Motor starts fine, than it revs up, then nearly dies,
then revs up, then nearly dies, etc etc. It -does-
carbonize spark plug electrodes ("black plug disease"),
and I thought that was the problem, but when I put a
fresh plug in, it ran the same as with the black plug.

Anybody got any idea what might be going on?

TIA,
Puddin'

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