The electronic ignition that you are running needs 12 volts to operate correctly. HX yellow wire is only about 8 volts. That's why the relay has been fitted. You need an oil filled coil for the HEI system. I managed to score a couple of these coils locally. They're getting a bit hard to come by. Make sure your relay is connected properly. Terminal 30 on the relay should be your 12 volt supply (should have an inline fuse) , Terminal 87 on the relay should go to the positive side of the coil, 85 and 86 are the switch terminals. One should go to earth and the other should have the original yellow wire connected to it.

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You can also get rid of that distributor cap by replacing the cap with a Bosch GB926 and the rotor button with a Bosch GB 786. You'll need to replace all the leads as well. Makes for a much neater engine bay.