318 missing what could it be

Take a socket and with the distributor cap off and rotor in place, rotate the engine clockwise until the timing mark is at the "10° BTDC" mark. Then, watch the distributor ignition rotor closely and rotate the engine counterclockwise very slowly while watching it. The second the rotor moves, stop turning the engine. Then look at the mark on the balancer. If the balancer moved more than 5° before the rotor moved you should look at the timing chain. I would also advise pulling the intake just for the sake of being able to look at the camshaft. You may have a worn lobe or two making the miss.