Big-time Mag Drop

I would double check wires to the bad module and check the work you did replacing what you called the ignition unit. Sounds like one of your wires came off.

Will do, Roger.  Thank you for the prompt response.  Will report today's findings.   J

Opened, inspected and cleaned all accessible ground connections to ignition modules.  Applied light coating of dielectric grease and checked continuity after reinstallation.  Opened up all four of those devilish plastic connectors at the modules.  They look very clean.  Have left them open so I can ask the Forum's advice on (a) whether light and careful application of dielectric would be advisable here; and (b) what kind of testing could be done with these little rascals opened.  Can my multimeter tell me anything?

I ran the engine after the ground connection work and the big RPM drop is still there, even at low RPM, so I'm moving on with the trouble-shooting.   Whatever the problem is, it doesn't appear to be related to the high RPM preflight check.   TIA   Jim

Have you swapped the ignition module connectors to see if the issue stays on one side or goes over to the other module?

Hi Jim

First, download the latest maintenance manual heavy for your engine type.  Go the electrical section and locate the ohm resistance chart.  You can check every  ohm value of the components.  The only thing that you can't check with an ohmmeter, (should be good quality auto-range type), is the modules themselves.  As Roger suggested change the A to B and B to A and run your test once more.  If the problem moves with your module then that is where your problem is. Be careful on which primary wire is connected to the coils and look for breaks at the connectors to the wire itself.  

I don't know what used part you got, be sure it was the same part number, that is marked on the side of the module.  The serial of the module will tell you what year it was produced, look at that part and the first 2 digits are the year, then a space and the last 4 numbers are the serial within that year.  it could be it is a very old part. (?) You can't be sure with a used part sometimes. 

Cheers