Hopefully he doesn't have Holley carbs on it, since with all that backfiring the power valve would be gettin abused quite a bit. I have seen it quite a bit on startup but not when running consistently at a steady load. There might be a leak in the carb (fuel, vacuum of some sort), or for some reason I am thinking that it is possible that the valve is not closing completely and he is getting some combustion to sneak back up the intake...how loud is this popping and does it sound sort of consistent, almost like it is one cylinder? However, that whole open valve thing is sorta rare, so my best guess is that some air/fuel mixture is igniting before it gets into the cylinder. I can only think of a couple of things...I am far too young to be any kind of expert on the subject. Hopefully someone else is more help to you.