Drop in RPM magneto test

Since different ignition modules didn't make a difference then I would suspect the issue is before the modules. You now need to check the trigger coils on the back of the engine. This chart is for testing wiring at the six pin connectors or It could be just a trigger coil gap that is too wide. Trigger coil gaps are .012 - .016. Make it the smaller gap and not the widest.

Heavy maint. Manual. Section 74-00-00 page 22

3.12) Measurement values for the ignition unit
The following measurement values can be checked at the appropriate point after
detaching the plug connections.

Generator coil  (on stator) yellow - yellow 0.1 to 0.8 Ω

Generator coil yellow - ground ∞

Charging coil  (on stator) red - ground 3.2 to 4.5 Ω

Trigger coil green/white - blue/yellow 115.0 to 123.0 Ω

Trigger coil new * (with clamps) white/yellow - blue/yellow 220.0 to 250.0 Ω

Primary ignition coil connection contact - ground 0.1 to 0.4 Ω

Secondary ignition coil high voltage - High v. 6.1 to 6.7 kΩ

Spark plug connector 4.4 to 6.0 kΩ

If the engine has that much drop at 4000 it may be a lean mixture, especially if this is on both.  Check what position the needle clip is in.  If it is a 912 then use position 3 and also inspect that your jets are not coated with varnish. 

Cheers

Thank you both very much, I'll get started on the task.

Hi RW,

Just curious,

That seems like a lot of rpm drop for one notch on the carb needle? If this was the case why wouldn't people who do go from the #3 to #2 slot to lean it out have the same issue? I wouldn't suspect the needle slot issue unless someone has been in the carbs recently and this all started right after that.

Hi Roger

No the issue is it is too lean not too rich.  If this is in the number 2 clip then go to the number 3.  Old Rotax engines from the 80s were stock at number 2.  They would be fine in the summer flying but when it gets cold in the winter they were very lean and very high mag drops will then happen.  You could have the same thing happen if you have a lot of varnish on the jetting, which will lean the mixture.  It is not that hard to check and 1000 RPM is a lot and there is no complaint about rough running so in my mind this is not electrical. 

Cheers