Spark issue

[Silverback]

I'm working on a 12.5hp powerbuilt briggs engine. I'm not a mechanic. Motor running rough and backfiring constantly. Replaced carb. Adjusted idle and airflow. Replaced spark plug etc. When I pull the boot off the sparkplug and hold it about a half inch away from it. The damn thing starts right up and runs like a champ. No backfire or rough idle. When I put boot back on sparkplug I can barely get it to start, and when it does, it's constant backfire and rough idle. Also, lights don't work. They do however turn right on when I'm holding sparkplug boot in front of sparkplug. I'm confused. Is the mag bad? Or do I have an ignition switch problem?

 

[bertsmobile1]

That is a bad plug
If you hold the plug lead that 1/2 inch off the plug for 5 to 10 minutes and the engine is not generating oil deposits or carbon deposits then it should then start normally.
That is the old school way to clean a carbon or oil fouled plug .