did you fit a new copper seal between the exhaust pipe and head, before you did up the 2x bolts?
if so, then it may just need you to tighten up the 2x nuts
this will seal the joint, and wont let air to get in and should stop it popping
it can also leak at the rear of the pipe, where it joins the muffler.
the rockers for the valves will most likely need adjusting like Blender suggested, but its a bit tighter than what he posted, and the other way around too.
intake needs .003" clearance and exhaust needs .004"
here is a video that might help you set them, but use the clearances i wrote above.
http://www.youtube.com/watch?v=QJciyJRJh8Q
carby flooding,
carby may have something stuck in the needle and seat?
and not letting the needle seal/close off into the seat when the fuel bowl is full
and is there something sitting between the intake manifold and the head?
there should only be a gasket there.
almost looks like there is a plastic insulator between them, like the one on the carby end of the manifold?