Engine Engine bogs down

1554 (27hp Koehler) starts fine and runs fine for a while but suddenly sounds/feels like it's loaded down. No blue smoke but get an odor of fuel and it seems to use more than usual. It almost seems like it's getting too much gas but not enough to flood it out. I've changed all filters and plugs. ALSO if I pull one plug, it runs fine. If I pull the other (after replacing the first) it dies making me think something in the cylinder????



Any ideas?