Remove the muffler or at least loosen the exhaust flange bolts. Run it up to see if it continues to pull properly. If so, your muffler is being restricted by a collapse inside. Doesn't happen often in the mower world. However, it does happen.
I actually tried running it briefly without the exhaust on to see if there is indeed a blockage and didn't change anything. There is also plenty of pressure coming out of exhaust.