A New Problem

I went to the hangar early as I had to remove the upper cowl and work on the RPM wiring. When the cowl was off I could also verify that the electronic ignition modules had not been exposed to excessive heat so far (this will change in the summer, I suppose).

So far I have always opened the oil door soon after turning off the engine and left it open to allow the hot air to escape. I was also careful to get the cowl out of direct sunlight when the engine was hot. All this might have helped keeping the temps down a bit.

Concerning the RPM wiring, I had prepared a 20K Ohms resistor with spade connectors to allow it to be inserted and replaced if necessary. The following picture shows the wiring you would want to work on. The black wire is ground and the white/green one carries the signal (and would get spliced for the resistor).

I spliced the white/green wire and added spade connectors at its ends and inserted the resistor. The resistor is covered with shrink tubing that connects to the spade connectors in order to reduce stress and vibration (and dirt).

With this fix installed I did a ground engine run with the cowl still open. The RPM was steady within the limited range of RPM I could produce on the ground. The static WOT run showed 4920 on the first attempt and 5000 RPM on the second. I could only verify in the air if this resistor fixed the problem, so I put the upper cowl back on and took off.

During take-off and climb out I still experienced somewhat jump RPM readings but as bad as it used to be. After throttling back the RPM was rock solid and I continued flying around 5300 for a few minutes before I leveled off and let the autopilot take over to maintain altitude. I open up the throttle to maximum again and saw steady 5520 RPM. I even descended a few thousand feet after this and did another WOT climb at 75 knots without any jump RPM readings.

I will see today if this fix solved the problem completely when taking off again and doing the climb rate evaluation test (which requires extended WOT climbs) and the speed test box at 5500 RPM.

Now what is the new problem? During the ground run with the cowl open and the subsequent flight I experienced very jumpy and fluctuating readings in the left EGT sensor. The right one was rock solid but the left one almost never settled and even dropped off the SkyView a few times. This might indicate that the sensor is dying or the wiring has broken. Thankfully, this sensor has practically no value as its only real use would be during mixture changes of the engine but the Rotax does not allow for manual mixture changes. I will watch this for a while and then decide if and what to do about it. This is certainly nothing that would keep me from completing the test phase.