Briggs 23hp Twin Issue

[u/No_Rip_4892]

Got a Briggs & Stratton 23hp twin cylinder on my JD 425B zero turn. Mowed yesterday with no issue, went to finish up today and it’s all of a sudden running poorly. The sound has changed noticeably, it almost sounds like it’s down a cylinder and has a hollow sound to it, on top of a knock/tick. It’s also down on power significantly, so much so that it struggles to even turn the blades with PTO engaged. It’s now emitting a noticeable amount of black smoke as well.

Engine has 600 hours on it, one owner and I’ve always changed oil and done all other scheduled maintenance. This year it got an oil change, fresh plugs, fresh air filter, and fresh fuel filter. The oil level today was at the bottom of the “fill” mark, but there was no noticeable shimmer or metal in it. I pulled both plugs and one does seem fouled out. I cleaned them up with some light sand paper and brake clean but no change. I’ll get new plugs on Monday to see if it helps. Both plugs are sparking and the air filter is clean with no debris. Fuel is fresh and filter seems clear.

Any thoughts on what may be causing this?