Another issue talked about is fuel loading up. Large ports result in low velocity in the intake track at idle. This allows fuel to drop out of suspension and form on the walls. Large droplets drag along and eventually drip into the cylinders where it washes
oil off of the cylinder walls, causing excessive wear.
Large cams with a lot of valve over lap exasperate the problem. At idle, the intake vacuum sucks exhaust gasses backward into the intake manifold, reversing the flow for a brief moment. In order to change directions, the velocity must go to zero, allowing fuel droplets to form. In addition to this, air can get momentarily pulsed backwards in the carburetor, resulting in some air going past the jets a second time. So there are small pockets of intake charge with double the fuel.
On extreme engines you will hear them idle, and over time the idle slows and the engine starts burning richer. If you let it idle for an extended time, when you rev it, it will act like starting a flooded engine with a WOT. It will blow black smoke and putt putt until it clears its throat. An engine like this should not be allowed to idle very long.