Briggs/Stratton 4 hp Quatro

On an MTD 21 " mower, maybe 8 years old.

For several years, even right after disassembling/cleaning
air filter, carb, changing oil, etc etc, it'll start up
and run steady for about 10 seconds, then it revs up and
down (almost killing) in an endless cycle. About 30% of
similar B&S mowers I see running in the 'hood do the
same thing. Oh, and, of course, it fouls spark plugs.

I cleaned it up last fall. Start it up this spring, does
the same thing, but, after reaching minimum rpm, it
clanks while trying to rev up again. Sounds like serious
pre-ignition or even a flogging rod knocking. Motor has
had no hard use: I mow my little lawn at an idle.

Anybody got a handle on such behavior? Smokes a little
only when first started. Compression feels OK.

Cheers,
Puddin'

"Life is nothing but a competition to be the criminal rather
than the victim."
- Bertrand Russell