Small Engine Troubleshooting Problems? Ask a Mechanic Now.
You didn't run it with exhaust off?
If there is no way the exhaust is plugged. Then it's too much fuel/too little air, or the timing is off. The carbon on the plug only comes from incomplete combustion.
Try setting the valves. .006 inch intake, .008 exhaust.
Then if no help, replace the flywheel timing key.