healthy & repeated backfiring thru the carb is one of 2 things:
1. A spark plug firing way too soon (like when the intake valve is open) - wires not routed properly.
2. A intake valve not closing, or open at the wrong time.
At this point, you need to make sure you are getting good even compression on all 8 cylinders. This will also tell you if the cam is lined up properly.
Do a compression test at this point.
Last edited by tjet; 11-13-2012 at 08:14 PM.
Reason: what 123pugsy said