Just rebuilt a 250 Trailblazer [not my first rodeo] Rebuilt from rejected nonrunning hulk. New top end, been all thru the carb and fuel system. It starts fairly easily with the pull start. Will not settle down to idle it just wants to rev up like a runaway diesel. Will not shut down with key or engine stop switch. The only way to kill it is to choke it to death. I’ve rebuilt many of these motors and have never seen this before