Ignition Check Irregularity

No, Roger, this problem is recent and unrelated AFAIK to the plug and cap replacement.  I will fly this morning and try and jot down more specifics.  I appreciate your thought that it might be one plug, but the on/off nature of its occurrence--particularly post-flight--runs contrary to my experience.  My airport is uncontrolled, and taxi time is pilot optional.  Has anyone ever had a similar problem that traced to those intricate little plastic thingamabob cubes that connect to the modules? 

When my first module died, it just went dead.  Period.  Died in its sleep.  A Rotax technician told me sometimes the modules fail progressively.  Does anyone think this rolling RPM drop is indicative of a dying module?  I don't mind throwin' $3 sparkplugs at a problem but a failing module after less than 200 hours...  Well, Austria is a long walk from Texas to give them a piece of my mind.    Again, thanks to all for your suggestions.   Jim

I am having difficulty with this RotaxOwner site's response mechanism, so if I am redundant here please forgive me.

Addressing your thoughtful responses: A solid "no" all of Mr. Griffin's inquiries regarding modifications to the plane.  The answer to Roger's questions: No, this problem came up nearly 100 hours after plug and cap replacement.  It is a recent phenomenon.

I flew the plane today--ambient about 50F--and noted the +-300RPM miss on Module B--before and after flight.  Minimal drop on A.  B is the new module.  This is the same, maybe slightly worse roughness I experienced before installation of the new module.

Before I start checking the plugs, I would like to explore any other possibilities y'all might have come across.  A Rotax technician once told me the failure characteristics of modules vary from gradual to complete no-go.  Does this comport with your experience?  My "A" module just plain died a couple years ago. Nothing...apparently died in its sleep.  I replaced "B" recently because it had over 1000 hours, but the roughness continues.

I'll pull the cowl and run it on the bad module and see if one jug is lukewarm.  Then check the plug and cap.  Beyond that medieval trouble-shooting, I'm out of ideas.  I don't think this is fuel and it's definitely only on the one module, which is a new one.  Maybe the wiring in those fussy little plastic cubes?   Thanks in advance for your thoughts.   Jim

OK, installed 8 new plugs, gapped to .025, applied dielectric grease as directed.  Warmed up to operating temperature and checked ignition modules at 4,000 RPM.  Same old, same old.  No change: 350-400 RM drop on one module.  Acceptable drop on other module.  Plugs looked good, some were a little sooty.  All gaps on removed plugs were .027-28.

One thing I didn't think of until I was driving home: try the choke at 4,000 RPM and see if that might somehow help.  That would indicate a mixture issue?  But would a mixture glitch just show up on one module?  As I reported before: the RPM roughness goes completely away once the idle is reduced south of 3,500.  This, to me, seems significant but I don't know where to look.

The cowl is off, so I'm ready to try anything y'all might think would work.  I can't imagine a fuel issue on just one module.  Ambient today was 70F.  The big question for me is: can my "older" module with less than 100 hours on it since new be misbehaving in this way?  As I reported earlier, the problem remained exactly the same after installation of a new module.   Jim

First, lets not blame the Rotax Owner website forum, it's about users helping each other as most forums are.  

On the 912 ULS the modules are the same top and bottom.  Simply swap them top to bottom.  If the problem follows the module then you know it is in the module and not the wires or plugs or plug wire ends.  There is a possibility it is a problem with a trigger but that is remote.  All the ohm resistance values are given in the maintenance manual heavy, you can check almost everything from that with the exception of the module itself.  (this is why I suggest you swap them to isolate the issue) 

As Roger has pointed out try new plugs, once you are sure what module is giving the bad reading that is only 4 out of 8 plugs at most.  It could be a high resistance of the high tension spark plug wires, that check is in the ohm readings.  Given you have tried new plugs look for a bad or loose connection of one of the coil to module wires or the trigger coils.  Perhaps when you swapped a module one of the connections is not working as it should. 

Mixtures, not likely if it works good on one module then the fuel system is fine.  That issue normally shows up when both have a high mag drop.  My views of course, you would need to do all the checks for sure. 

Cheers

Unscrew each spark plug caps. Cut the wire back about 3/8" to get some good clean wire. Put a dab of dielectric grease on the end of the wire and screw it all the way on until it stops. You mentioned yo put dielectric grease on the plug threads. Don't do that. It is a white colored thermal conducting paste. DO NOT get it down on the plug tip.