Ok generally speaking most engines are not very sensistive to having to much fuel. The lower the compression ratio the more this is true. Higher compression ratio engines are more sensitive to overfueling. I imagine this is becuase it is harder to light a fuel heavy mixture with lots of cylinder pressure.
Now what will absolutely kill power is being to lean. 1 point to lean can kill output by as much as 1/3 easily. To rich though and the percentages shrink greatly.