Feels like my tractor is only at half the power

I just changed out my cylinder head and rods. I cut grass for about 1 hr and now it seems like I'm only getting half the power. I took off valve covers on both sides and all looks ok. 1 spark plug was dry and jet black with soot the other was damp and gray looking. I have a Briggs & Stratton 24 hp els725 ohm engine. Muffler sounds a little muffled too. Engine sounds weak not like when running good. Any ideas.