Murray/Briggs missing/popping

[Peavler]

I have an old but good Murray 11hp Briggs that starts off the key but misses and runs bad,til I unhook the yellow wire (key) from the black wire that goes to the engine,Then it runs great however with this wire unplugged I cant kill the engine off the key switch,I can plug it back in and shut it off or I can just choke it til it floods out. What is the problem here,Is it the key switch ? This is a 1992 11/38 Murray with 4 speed and no brakes