On my van w/318, auto and carb. I've replaced ballast resistor, spark modular, starter relay, pick up coil, ignition coil, and voltage regulator and the starter its self. Now the only spark at all is a dull yellow one that only shows when the ignition is turned off, it jumps, confused!
Check voltage to the ignition coil + terminal when the engine is being cranked over (should be battery voltage; 12v). Also check the other side of the ignition coil with a test light, the light should flash as the engine is cranked over. Let me know the results.