912 ULS with 460 hours built in 2008 1000 RPM drop during Mag test

Check your mag switch and it’s connections too.

I can check them again, but... wouldn't a bad connection mean that there would be none or almost no rpm drop because the switch is no longer grounding the ignition to ground?

michael Vadney wrote:

The main set of 6-pin plugs for the ignition black boxes is what I swapped first.  The 1000 rpm, rough running problem still occurred on the left mag-kill switch.   That should tell me the the problem is upstream of the ignition boxes.  So no, I have not tested the plug-caps, wires, or coils.
Am I mistaken?  If the problem was one coil(s) then the problem would have jumped to the other switch I would think.

Sorry if I'm being dense but I'm still not clear. Do your modules have 2 x 6 pin connectors or 1 x 6, 1 x 4 and 1 x 1?

If it is the 6/4/1 type (the age makes me think it may be) and you have swapped ONLY the 6 pin, then I think you have not tested the module itself or the trigger coils. You have tested the HT coils/leads/plugs.and the magneto coils.

The left kill switch - the 1 pin connector - is still operating the same module, which is still using the same triggers. However that module is now using the other mag coil and the other HT coil etc.

If you now swap the 4 pin plug (as well or instead) but don't swap the 1 pin, and the problem changes sides, it must be the trigger circuit. If the problem stays on the same side, it points to the module.

Sorry if this is wrong, it is confusing!

Good morning Mike.  Thanks for asking.
There are two 6-pin plugs per igniter box.  I believe that is the "new" style.  The ones that I swapped were between the two inputs to the black boxes.  The lines coming from the stator and the triggers.

In the Heavy Service manual (attached) the yellow highlights are the two plugs that I swapped.   I also don't believe that it can be a bad "mag" switch either in that if the switch did not work and connect the circuit to ground then I would have no drop at all in rpm.

The engine otherwise starts and runs very strong.   I am also fighting a hot cylinder issue as well.  The number 4 cylinder is like 200 degrees hotter than 1 and 2  I believe that it is an airflow issue and if I yaw my gyro (pusher prop) to place more airflow on #4 then the temps come down.  I am making some carbon fiber ducts this week for the 3 and 4 cylinders.   I do not believe that the two problems are related.

Thanks!

With the clarification of what you have done, I have to agree with your logic. You seem to have isolated it to the trigger circuit. Strange that the resistance measurements are ok though.