Did you measure through the plug cap or just on the plug wire? If using 5K ohm caps, that would be added to the 8K ohm coil value for a total of around 13K. Yes, it should spark with that. The original caps were rated at an oddball 8 or 9K ohms so the total with them would be even higher, and the bikes ran. Are you sure that aftermarket ignition is wired in correctly?