Few months ago my airplane was having some high RPM drop during runup. It was still under the acceptable drop, but it got me worried since it is a new airplane, purchased brand new last year (I even posted here at that time). It resolved itself.
Now, at the 50h inspection, I removed the spark plugs and they all looked good. Also did a compression check and all cylinders were very good (86/87). I was recommended by an experienced Rotax mechanic to clean the old heat paste and reapply to each plug, which I did. I took the Rotax training and few comfortable on doing it. Put the spark plugs back into the engine, synchronized the carbs, and went for an ops test. Without the cowling, did a runup and the RPM drop became worse! At 4000RPM I was getting about 400rpm drop on the RIGHT switch and 200 on the left (it is a key-type ignition switch). So the airplane was grounded.
I then decided to replace all spark plugs, all brand new original Rotax. Tested again, no changes, same problem. Uhg…
Then I swapped the horizontal connectors with each other (to my understanding, those are the ones that connect the coils to the modules) and did another runup. The 400rpm drop then switched sides, now showing the drop on the LEFT switch. To my knowledge, this means that the issue is downstream of the coils, meaning either the module or cables/boots. The spark plugs I ruled out since I had replaced them by new already with no changes.
Then I switch the connectors back to original and swapped the vertical connectors this time. The problem also switched sides, showing the rpm drop on the Left switch (remember, the original drop was on the right switch). If I understand the electrical diagram correctly, that means the problem is not the modules, but on the cables/boots. Right?
I measured the resistance of the boots (circuit boot-coil-boot) for each pair, e.g. 1T w/ 2T, 3B w/ 4B, and so on… and got all of them in the range of 15.6 to 16.6 kohms, which appears to be what is normal/expected.
The other thing I noticed is that if I just leave the throttle at a given rpm, they keep fluctuating +/- 50rpm, sometimes more. Also, when doing these mag checks, I always did it 3 times every time, and at least 1 time the results would come normal, but the other 2 would show the 400 rpm drop.
What you guys recommend as far as next step on troubleshooting? Are my conclusions above correct? It seems so non-conclusive… could be carb sync instead? (I did sync the carbs, but maybe messed it up?)… Engine is still under warranty. Appreciate any help.
