Bogging down means the engine sounds like it's trying to do the work but it just can't. Like cutting wet grass with a lawn mower. Or up-shifting into 5th gear instead of 3rd with a car.

Both problems will cause that symptom but they sound different when it happens. It's easy to tell which is which. If your pulling 12 degrees of pitch and it bogs down while the rest of the flight envelope is O.K., you know it's too much pitch. If your engine responce is too fast (makes funny things happen) and while hovering and it bogs down after X amount of time while hovering, your too lean. If throttle responce is poor and your engine bogs down while trying to keep up with pitch changes, your too rich.

Don't take this like I'm an expert becase I'm not the best when it comes to RC engines, but this is what I belive it to be.