David,
Pull the valve cover, look for anything obvious (broken pieces), pull the coil wire or unplug the HEI depending on which type of ignition you're running, have a friend bump the motor over, on each cyl, make sure they all are opening about the same (a wiped lobe won't open as much or at all), after the intake valve closes, check the rocker arms for loose (loose nut/broken rocker/bent push rod), see if the valve spring can be rotated (it shouldn't), try to rotate the push rods (they should spin true), this should put you on track.