I recently bought a used set of RC SL9-310 Injectors. (310cc) The connectors that came with them have two black wires comming off of them. Therefore I have no clue which one is the common wire. My first question is if anyone knows which one it is. Second question is if it will hurt anything if they are hooked up backwards.

 

As long as all 4 are connected the same way it doesn't matter. Injectors don't have polarity.

 

So the common and signal wires can be switched around, as long as the other 3 are connected the same way?

 

Correct, just make sure all 4 are wired the same way and you're golden.