ignition check?

The blackness/ or richness is comming from the port volume being larger, and the 318 bore can't use that much port volume and the velocity is slower due to larger ports and valves. Which when the port becomes too large the fuel ratio will be richer as the engines vacuum is lower and the signal to the carb is stronger which it thinks that the engine needs more fuel. Smaller ports that are more efficent will make more power and torque and will be easier to tune and use less fuel. This is why the factory made 2 sets of heads, one for the 273/318 and one for the 340/360. The only reason that they use the same heads on the 318/360 magnum engines now is because of the fuel injection systems. And they change the computers to adjust the amount of fuel that each engine recieves so that they can maximise hp and tq. and meet emmissions.