Briggs and Stratton running rich

I have a 6.5 HP B&S Quantum edge motor (123K02) on a pressure washer. It is about 6 years old with low hours (maybe 15-20) It runs , but at full throttle it stutters some. pop.pop.pop...Intermittently. As well, my plug (around 4 hours on it) was sooty. It has a manual choke carb with no adjustment screws or diaphram. The air filter is very clean. I cleaned out the carb with the normal carb spray/ wire through the jets. It was fairly clean, and the jests were not clogged. I put it back together, drained and dried the gas tank and added fresh gas.. Installed a new plug. It is still making the popping noise. Inspected the throttle and choke plate, they are wide open as they should be at full throttle (no load from sprayer). Compression is 55-60lbs. If anyone has any ideas, I would greatly appreciate it. Thank you