A cold front went through on Friday, and the wind was really blowing on Saturday - gusts to 35 kt in Ottawa, and it looked similar in Smiths Falls. The winds stayed high all day, so I didn’t even consider going flying. I went out to the airport for the morning, to get the engine data recording hooked up and tested, and to troubleshoot a VHF COM issue.

I had discovered a problem with COM 1 (a GNS 430W) during the previous aborted flight attempt. There was no TX indication on the GNS 430W when I attempted to transmit. I switched to COM 2 (a Microair 760) - it worked fine.

On Saturday I pulled the GNS 430W from its tray, and used a multimeter to see if the Push To Talk (PTT) signal was making it to the radio. I was very happy to discover that no PTT signal was present at the radio, as that meant the problem wasn’t in the GNS 430W. I knew the PTT signal was making it to the PMA–4000 audio panel, as COM 2 worked. So the problem was either in the audio panel, or in the wiring between the audio panel and the GNS 430W. The next logical step was to check the wiring harness between the audio panel and the radio, but I had forgotten to bring the installation manual which would tell me which pin in the connector to check.

This morning I went back to the hangar, armed with the connector pin-out for the audio panel. I had woken up in the middle of the night with the realization that there was a connector in the wiring harness between the audio panel and the GNS–430W, and that this was a logical thing to check. I crawled under the panel this morning, and found that my problem was self-induced. I had pulled the autopilot from the instrument panel in August to send it back for a firmware update. I had disconnected the connector in the harness between the audio panel and the GNS 430W so I could move the harness out of the way to allow me to get the autopilot out, and I had neglected to reconnect it. I hooked it back up, and a radio test showed everything was now working.

Next I did two high speed taxi runs, accelerating up to 80 kt, then rolling to a stop at the end of the runway. The engine pulled strongly the whole time the throttle was forward, with no hint of the drop in power that I had seen the previous weekend. I looked at the recorded data over lunch, and everything was completely normal.

After lunch I went flying. The engine behaved perfectly, with no anomalies at all. I did a 2.5 hour flight, following the instructions in Lycoming Service Instruction 1427C. I ran at 75% power for one hour, followed by an hour with the power alternating between 65% and 75%, followed by 30 minutes at full throttle and max rpm at 2500 ft. The only issue I noted is that the prop governor max rpm stop needs to be tweaked, as the rpm during take-off was about 50 rpm too low during take-off and about 100 rpm too low once everything was up to working temperature.

The next step is to pull the cowlings and do a full inspection, and adjust the prop governor max rpm stop.

I’m still somewhat troubled in that I never found a definitive cause for the engine issues I had last weekend. I found and fixed a few things which might possibly have caused the problem, but there is no way to know if any of them were the cause. The three leading candidate causes are:

  1. Electronic ignition firing at wrong time due interference from spark plug leading touching coax cable between electronic ignition control box and ignition coil.
  2. Fuel valve not fully open. I had changed fuel tanks prior to the runup, and maybe I didn’t turn the fuel valve all the way to the stop, resulting in a flow restriction.
  3. Bad quality fuel - the last fuel I purchased before removing the engine was in Yarmouth, NS in early July. Yarmouth has very little flying activity now, so it is possible this fuel had been in the storage tanks for many months, followed by almost four months in my tanks. I had to wait quite a while for two aircraft to land downwind before my attempted take-off, and perhaps it was warm enough in the engine compartment for bad quality fuel to vapour lock.