Dont assume your old injector pump is set correctly.
My father inlaw had 1 reconditioned only because of the old suphur leak problem, reinstalled it and it destroyed the motor.
( it was a toyo so that don't matter Hay!)

If the motor was fiddled with prior to you getting it, and it sound like it may of been "chipped etc", you have alot of home-work to do to prove it's their fault.
Don't like sounding doom and gloom just trying to think what may need looking at first.
Hope it works out for you.