Not enough air to burn cleanly all the gaz that is injected, so you lose all that potential power also making that blackish color which is simply carbon deposite which will end up making a higher C/R ( compression ratio ) making an even richer evironment.... in other words, it's not good.
The opposite is still not good. Too much air in the air/gaz mixture create more horsepower, but it hurt the engine because of the excess heat that is created. Burned valve is an example.
Hope it help.