B&S 18.5 hp engine won't run at full speed/power, exhaust from air inlet

Hi guys - not super experienced with small engines, but I bought a new-to-me John Deere Z225 riding mower (2011 vintage) with an 18.5hp B&S engine that's having problems. Hoping for some ideas from you experienced folks.

It seemed to run smooth initially, but then I noticed "bogging down" every 7 seconds when unloaded. Replaced all fuel, new (chinese) carburetor, new (chinese) ignition coil, and measured RPM - it won't rev above 2350 RPM. (I don't think it ever did, but I wasn't attuned to that previously. Runs smooth up to 2350 but then as I advance the throttle it gets really loud and I see puffs of white smoke coming out the air intake every cycle and dark smoke out the exhaust and it just sounds like it's almost backfiring every cycle. Manual says it should go to 3300 RPM, but it just won't get there, even with full throttle. I did a dry compression test - 100 psi - seems good. Have not done leak down test. New spark plug got very black almost right away. Flywheel key is clearly aligning slots in crankshaft & flywheel so ignition timing should be good. Piston TDC is at about where the magnet is exiting from under the magneto,

Starting to wonder about valve timing. See video. Very mystified. Any thoughts?

TIA!