I recently bought a Cub Cadet Ultima ZT1 54 inch cut. I mowed approximately 2 hrs on it and shut it off. The next week I started it up to mow and I noticed that when I engaged the blades and pushed forward on the levers to move, the engine bogs down like the grass was choking it down. I have changed spark plugs with proper gap, cleaned carb, cleaned fuel system, checked mower deck for any binding debris and it still chokes down. Anyone have a clue what might be wrong with this thing? It's really frustrating because the mower is brand new and I can't mow over a crawl speed.