A couple of years ago I rebuilt a Rochester that i had sitting around and installed it on my 216. I suddenly had all kinds of power that I didn't have before. I then puleed the plugs and they were all covered in carbon, which they weren't before. I looked at the jet and it was stamped "58". I found one with a "51" jet and put it in. I lost some power but my plugs returned to the grey white color that they had been before.
