On my way to the Copperstate fly-in on Friday, I stopped in Bishop to pick
up Kevin Klinefelter. We heard a loud, irregular popping noise in our
headphones while taxiing to the runway, and the runup showed one rather
rough mag. We checked the plugs and wires, finding no obvious problem with
the wires, but the four plugs in the front cylinders were all very black,
though they had only a few hours on them. (The rear plugs were all fine.)
All plugs were replaced, and everything seemed smooth, including a standard
runup. (During the troubleshooting, the engine had gone from very hot to
very cool, with the cowls removed.) The popping noise began again on
takeoff, so we returned to Bishop, and I dropped off Kevin. With poor
weather in the forecast for the next day, I headed for home (180nm across
the 13000-foot peaks of the Sierra). After a couple of episodes of very
loud popping during the climb, everything quieted down again, so I continued
on up to 15500 feet (912S/Airmaster power), crossed the mountains, and made
it home without further incident.
There are four ignition coils on the 912/914 engines, with each coil firing
two cylinders (one live, one with valves open) at once. I measured the
resistance between the pair of plug wires from each coil, and found 17K
between the top plugs, and open circuits between the bottom plug pairs. The
suspect coil (based on the rough mag and the black plugs in the front
cylinders) is the forward coil on the port side, which fires the bottom
front plugs. It is nearly impossible to see anything in the tangle of wires
and hoses in this area, so I tried to remove the coil. After much struggle,
I got most of the way to the coil, but determined that it would be necessary
to either remove the engine from the ring mount, or remove the starboard
side intake manifold to get the coil out. Access to many parts of the
engine is difficult, but this is ridiculous.
Each plug wire is sheathed in a rubber sleeve, which covers the "tower"
where the wire enters the spark coil. Two of these sleeves are broken at
the coil end. Does anyone know how to remove the spark plug connectors from
the wires, or the wires from the coils? At least one of these must be
necessary, in order to get the wires through the sleeves, and also through
various other sleeves and holes. I suspect that the connectors may unscrew,
but I'd hate to damage one by trying the wrong thing. The coils come with
wires attached (at $280 each), but the wires and sleeves are also listed as
separate parts. It seems likely that my problem is a wire that is sparking
to the engine, or possibly sparking across a break in the coductor, but I
can't figure out how to remove them for further testing or replacement.
If anyone has experience troubleshooting or repairing the Rotax ignition
coil assemblies, I'd certainly appreciate any advice.
Dave DeFord
N135TD
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