Ignition Check Irregularity

No, Roger, I did not put dielectric on the plug threads.  I used the white thermoconducting paste.  As the wires are fairly new, I did not trim the wires as you suggested.  The mystery continues, however.  Just as an experiment, I removed the air cleaners and ran up the engine for RPM check.  Surprise!  The miss disappeared.  I ran it for maybe 20 seconds on the bad module, getting only 150 RPM drop, shut everything down, after double-checking both modules.  An hour later--of course--the miss reappeared...exactly as before.  Ambient temps mild, 70F.

I am becoming convinced the problem is module-related and am now in the process of removing the now-older (100 hours) module and replacing it with the module (1000 hours) I removed initially, as a test.  If that resolves the problem in a satisfactory manner, I will order up a new module and perhaps have a word with the supplier of the defective one.

If the miss persists, I will follow Roger's instruction and clip the plug wires.  I am mystified by the erratic characteristics of this issue and have been careful to maintain stability with all variables during testing.  I have complied with the advice of Lycoming/Continental hangar-mates and checked for induction leaks, etc-  I will report the results of today's experiment and welcome any constructive suggestions from the Rotax 912 community.    Jim

 I doubt it's a module. Bad ignition modules are either bad or good and not erratic or only 300 rpm drop. Usually 300 rpm drop is either a plug or wire and usually one plug not two or more. A bad wire can be erratic. Just because it's fairly new doesn't  mean it can't be a wire. If a plug cap was pulled too hard it may have caused the issue. Leave no stone unturned in your diagnostics. 

If the other suggestions don’t pan out, it’s possible you have one failing coil. Ignition coils that are starting to fail often work when cold but break down when they reach operating temperature due to the expansion of internal components and degradation of insulation, leading to an intermittent or complete loss of spark. This could account for the pre and post flight difference.  

Each ignition module drives two coils so it would be easy to narrow the troubleshooting down to a pair of coils by observing which ignition circuit has the larger RPM drop. You could buy one new coil for testing, or swap coil positions to see if the problem follows a particular coil over to the other ignition circuit. If it was me I would buy one coil and try it in both positions of the ignition circuit that has the larger RPM drop.  

Hi Jeff,

But if one coil was bad that runs two plugs and the mag check rpm should be higher than 250 - 300 rpm. Usually this rpm drop is associated with one plug.

p.s.

I like discussions like this. This is how we all learn.

Roger,

Probably, but if the coil is only missing a few ignitions on two cylinders it’s might only be 300 RPM. I think if your suggestions don’t solve the problem it might be worth a look. Remember what Sherlock Holmes said “When you have eliminated the impossible, whatever remains, however improbable, must be the truth“ 

I like these conversations also!