Almost Blew engine
There's only so much that will allow an engine to run at very high RPM and any way you spell this it comes down to A MASSIVE AIR HOLE or vacuum leak. Doesn't matter if it's an open vacuum port, PCV valve failure / broken hose/ forgotten, or throttles open, it has to be one of those things PERIOD
Take a REALLY good look at ports on the manifold, ports on the carb, condition and fit of carb base gaskets BOTH on the manifold AND on the carb, and look especially for some gap such as a base gasket not bridging some gap in either the manifold or carb
One popular problem EG are aftermarket "dual pattern" spread bore manifolds