Big-time Mag Drop

To recap: 912ULS, 1100 hours RV-12    Late last year my remaining original module, long in the tooth at 1100 hours, began dropping below minimums on runup.  I dutifully ordered a new unit, waiting several weeks on back order, installed the new one when delivered and flew several flights totaling two uneventful hours, with satisfactory runups prior to and after flight.

Subsequently, a postflight runup revealed a severe 3,000RPM drop on the new module, prompting this thread.  In troubleshooting mode, I replaced the sparkplugs and executed Roger's procedure on the relatively new plug wire ends.  No bueno, still a huge RPM drop.

As a classic troubleshooting procedure, I reinstalled the elderly, previously-removed module and flew several trouble-free hours, with the borderline RPM drops on the old unit, before and after flight.  This led me to conclude the problem was with the newly supplied module

My USA Rotax supplier initiated warranty procedures and I received a replacement module, which I installed Wednesday, checking carefully as I did so.

Today I pulled the plane out of the hangar, sans top cowl, to check the modules at warmup RPM.  All looked good with drops nearly identical.  Cowling it up, I taxied to the runup area and when engine temp got in the green, I tested the modules at 4 grand.  OAT about 70F.  RPM drops were similar, but I had hoped for better with the new one.

After a 20-minute flight, I checked the modules after I cleared the runway.  The new unit registered a +-1,200 RPM drop on several tries.  

I am mystified.  I am 99% certain I have not mixed-up A versus B modules in my testing.  It IS the new unit circuit which is registering the drop.  Is there something downstream in the ignition circuit which could be the glitch?  Since it ran predicably on the old unit, I can't imagine this.  Is there a heat-related issue, inasmuch as the big drop showed up post-flight after an acceptable preflight test?  I have heard of wires separating under heat.  Anything in 912 archives on this?

AFAIK, plane's fuel system is fine, with good carb balance.  If this was a '56 Plymouth, I'd be looking at the spark coil, the ballast resistor and the point gap.  I'm out of ideas with this Rotax.  I welcome knowledgeable suggestions.  Keep in mind, plugs and wires are very new and quite new, respectively.    Jim in Texas 

3K rpm drop as I said is not a single plug or even two plugs. You may have a bad wire to the module or a trigger coil issue on the back of the engine.

Roger, trigger coil sounds expensive and probably beyond my skill level.  Do you have any procedures I might use to isolate the source of the problem?  I am curious, however, that if it is a trigger coil, why does it not manifest itself when I use the older module?  That is, when I pulled the troublesome new one and reinstalled the old one.  Everything was fine then, albeit the marginal RPM drop on the 1,000 hour module.

Does the fact that the replacement unit functioned adequately pre-flight but exhibited the big drop post-flight suggest anything?   Seems to me that might indicate something heat or vibration related.  

Very much appreciate your suggestions on this vexing issue.     Jim in Texas

It could be a trigger coil gap is too wide. It just needs an adjustment. Is it possible a bad trigger coil, yes. In the Heavy Maint. manual there is so troubleshooting checks using an ohm meter. I would also check all my wires on top of the engine to make sure nothing has worn through the insulation.

Does the engine quit with this 3K drop?

Thank you, Roger, for the troubleshooting leads.  I will get out there tomorrow and follow up.  No, the engine did not quit with the 3K RPM drop, but it sounded like it wanted to.  I was too scared to keep it on that sick module for more than a few seconds.  I can try the second replacement module (in plane now) and get you something more specific on that one.

You may be on to something with the frayed wire idea.  On both the apparently defective modules, the pre-flight runup was OK; the problem manifested in the post-flight check....kinda like something worked loose in flight.  I will report with more specificity when I return.     Jim