Causes of slow spark?

Without a voltmeter you might jumper AKA "hot wire" direct from a battery source, such as the starter relay stud to the ignition module / coil.

Other thing you NEED to check is the distributor pickup connector.

Last, set up a test gap with the coil wire and crank it long enough to "catch a rythum". See if it seems like it is evenly sparking as the pickup would trigger it, IE steady, no large gaps in the rotation

I'm thinking either voltage supplyproblem, or a poor connection in the pickup wiring