Rotax 503 SCDI RPM drop when checking ignition in preflight

[leithalweapon]

I have a factory new Rotax 503 SCDI with about 30 hours on it. It all of the sudden is getting an almost 600 RPM drop on one of the coils when I do my pre flight run up. The other coil gets 300RPM drop which is normal. The engine runs perfectly with normal operating temps as per the manual. Also runs smoothly on the coil that is experiencing the 600 RPM drop. I have talked to two mechanics that have never heard of a problem like this. One mechanic suggested changing plugs and caps. I did this, but no change. The other mechanic said not to worry. It does worry me because the manual clearly states that the drop should be no more than 300 RPM. This stinks because I don't like flying behind a two stroke to begin with.

[comperini]

Are you sure the engine is really dropping 600 rpms, or is the tachometer just indicating 600 lower?

[leithalweapon]

When switching between coils I can hear that the engine is running considerably slower on one coil than the other. I doubt the tach is off by 300 RPM only when running on the coil that has the bigger drop. I will check it with an optical tach, but I doubt it will show much difference. It's worth a try.

[michaelheit]

Have you checked for any possibility of something making a short circuit path on the wires for that coil? If there is a resistance path created by oil, carbon, or some other conductive contaminant it could be causing a greater RPM drop on that system. It is also possible that there could be a problem with the section of that systems components. If at all possible switch the transformer coil mounted outside the engie with the other cylinder and see if the problem follows that coil; if so replace it. If not then the problem may be internal on the stator circuit.

Mike Heit
Harrington, WA.

[bryancobb]

If they are long enough, switch plug wires "A" to plugs "B" and plug wires "B" to plugs "A" and run it. This may identify something.