12th August 2012

Tracking down the bugs

Hours: 2.2 | Posted in Flight Test

I had Allison come out to the hangar this evening to help me look into an engine stumble problem; based on EFIS data logs, the leading theory is that the #2 cylinder is having an intermittent ignition problem, and going dead occasionally at lower RPM’s.  So we ran down all the ignition wiring and did the troubleshooting steps in the Pmag book:

  • Applied ship’s power to each ignition in turn and watched that the LED indicator stayed on, while I wiggled all the wires and connectors from the power buss out to the ignition.  Both checked ok.
  • Ungrounded each ignition in turn and checked that the P lead did not ground, while I wiggled all the wires and connectors from the ignition to the ground block.  Both checked ok.
  • Checked the connectors and wires at each ignition for security and tightness…all ok.
  • Put a meter on the #2 ignition leads from both ignitions, checking for the correct resistance per the Pmag book…ok.
  • Wiggled the ignition leads and ends while watching the meter for a change…all ok.
  • Pulled the plugs from #2 cylinder, along with one from #3 for comparison…looked similar, normal and ok.
  • Removed the injector line from #2, verified that both the restrictor and injector body have no blockage.  All ok, reinstalled injector.
  • Unfastened all injector lines and directed into cups, conducted another flow test…all four lines flowed approximately equal volume.  If anything, #2 had a tiny bit more fuel than the others, so it doesn’t look like it’s a fuel restriction.
  • Inspected the airbox and fuel servo inlet for obstructions…nothing visible.

At this point, I can’t think of more to do with it right now except pull it out and run it up, taxi around and see if I can make it stumble again.  If it does, I’ll test the ignitions to see if I can isolate it to one side or the other.  I could also try replacing the plugs on #2 in case one is internally damaged and not firing consistently.  Raining tonight, so no going outside…hopefully tomorrow evening will be better.

On to the next problem…the weak radio transmit and receive.  The AWOS has always been faint and scratchy inside the hangar, and I just attributed it to the two layers of steel walls, but the transmissions yesterday weren’t right, and the AWOS in the air was weak, too, even at 5 miles from the field.

  • Swapped the connection at the radio from the #2 antenna and feedline, to the #1.  No change.
  • Back to #2 antenna and line…Removed the #2 antenna and doubler, removed the primer from the belly skin where the doubler sits, scotchbrited and cleaned both, and reinstalled.  No change.
  • Tested the feedline with a meter for shorts…none, tested ok.
  • Fiddled with the configuration settings in the radio.  No change.
  • Had Allison go outside with my handheld (which I’ve been listening to traffic on, inside the hanger, for the last few months, without any problem, receiving transmissions from this airport and others).  When right outside the hangar, it was fine, but she lost contact at the end of the hangar row (a few hundred feet away).
  • Also observed: sometimes when the mic is keyed, the sidetone isn’t normal sounding, it’s very weak and distorted, unintelligible.  Unkeying and immediately rekeying the mic will make it sound normal.
  • Also observed: the sidetone level setting seems very low; anything above 20 or so (out of 255) is deafening.  The factory default was something like 170.  I think I’m set at 17 now, and it’s quite loud, louder than I want it.  Turning the sidetown level down too far causes it to become quiet but very distorted (aside from the occasional distortion on transmit above).
  • Also observed, but unrelated: the “radio mutes music” key on the audio panel doesn’t seem to work properly, the radio is always muting the music, whether the function is on or off.

So either I have 2 very poorly performing antennas, or there’s something odd in the radio or the radio install.  The only part of the antenna system that remained common to both antenna tests, is the short whip from the radio tray’s right-angle BNC, to the BNC-F that the antenna feedline connects to.  I’ll get back under there and pull that part to inspect for bad connections, just in case.

Another potential test is to secure a different type of antenna from someone, mount it to a big piece of aluminum on a sawhorse for a ground plane, and connect it to the radio with a coax line for testing.  The replacement antenna would be a Comant CI-122 or the like, which has a different mounting pattern than the antennas I have now, so I’d want to prove it out before drilling a new set of holes in the belly and making new doublers.

I was hoping that removing the primer would do the trick, so I could put the seat pan back on; it’s a structural piece so can’t fly with it removed, and the quantity of screws make it a real joy to keep taking off & on again.  Alas, that’s the way it goes.  So, nothing resolved tonight, but hope for the future…if tomorrow night has decent weather I should be able to get it out of the hangar and do some ground testing on the engine problem.

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