This is a test to see if it is running rich. YOUR tail pipe is running black suggesting rich.
Warm it up then pull one of the vacuum ports open to introduce more air into the manifold. Like on the 1406 the driver's front vacuum port below the butterflies going direct into the manifold.
Or pull the power brake vacuum hose off on the rear of the intake manifold.
If this test speeds up the engine and smooths the engine out. Then you know it is running RICH.
