912 excessive RPM drop on lane A

Robert and Bill, here is a schematic of the ignition module with parts list and pics of the board dated 2015. It includes pics of an connection adapter loom for installing the newer modules.

https://drive.google.com/drive/folders/0B6ytVYfsfAiNflZzQnhkMG5SMllWWDlDdm5zX3RiZ3doU0E3T2lWRk5jNjdJaldqUW1MbUk

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Kudos go to Garrett !!!  

Having looked at the schematic, I can see that a single diode failure or even more likely a fractured solder joint on a steering diode could make it miss the leading edge pulse and fire on the trailing edge causing the delayed timing shift.

It actually appears that it might even double fire every time, or at least double trigger.

Of course, if the first 26° BTDC trigger worked, the circuit will not have recharged so the second 4° BTDC trigger does nothing.

And, since the fuel would already be burning there would be nothing to ignite anyway.

It might just be a failsafe feature that adds to the reliability but only cost $0.25 in parts and then someone else figured out that if you purposely killed the first trigger you could add a soft start feature for only a buck more. 

I put the new carbs on.i didn’t sink the carbs because it started up right away and ran smooth. I couldn’t have been to far off for testing purposes. Same results when toggling B. I’m driving 4.5 hours away to borrow modules. Wish I were flying there. I’ll keep everyone informed on the module test,?

This is a great day. I picked up a used pair of modules,installed them last night and problem fixed. From my experience if you swap the pin connectors on the modules and the rpm drop doesn’t change , don’t rule out the modules as the problem. For example I have the 6 pin, 4 pin connector set up. The modules are wired through the 6pin connectors. The excessive drop in rpm occurs when I toggle (earth) the b ignition. I reversed the 6 pin connectors and expected the rpm drop to occur when l toggle A ignition. To my surprise,it didn’t. The excessive rpm drop still occurs when toggling B ignition so I ruled out the modules as a problem, six months later I have been everywhere on this engine, I have even found things needing attention non related to my problem. In the end in my case it turned out to be a module. Thanks everyone for your help.

Do not get ahead of yourself.

Swapping the original modules and not seeing the problem swap sides is indicative of the modules NOT being the problem.

Seeing the problem disappear with the new modules only means that you have removed the problem but Not that you have identified the problem.

You have not identified the problem until you can Re-create the problem.

You have previously demonstrated that Either Original Module will operate correctly in the "B" Position and...

Neither Original Module will operate correctly in the "A"

Swap your old modules back into the engine one at a time into Both the "A" and "B" positions to identify which module has the problem.

If Both work in the "B" position and Neither Still work in the "A" position, the problem is the Position and NOT the Modules.

If neither module re-creates the problem, then you have probably fixed some other problem during the swap.

The number one suspect would be the wiring harness in general and the connectors themselves in particular.

The old troublesome connector on the harness may just be making better contact with the connector on the new module.

Have a close look at the connectors, and then have a closer look with a 10x magnifier.

A firm tug on the individual wires may also reveal something.

Until you can re-install the bad module and see the problem reoccur and follow the module, you can not say for certain that the problem was a module.