Reference Rotax 912 mounted on Ultra-light in Pusher configuration: Engine low time not equipped with Transmission Clutch or with soft start module.
Symptoms: Staring problem,-
-1- When engine is started with the throttle at idle position it runs rough and vibrates. If the throttle is advanced the vibration gets worse and the engine must be shut down.
We tried to start on one mag or the other or both and the symptom is the same.
-2- if the engine is started with the throttle at approximately 3500rpm position, then the throttle is rapidly retarded to idle as soon as the engine fires then, the idle speed is approx. 1800 RPM and everything seems normal, Mag checks are normal also.
What was done: we replaced spark plugs, did a compression check, replace all hi voltage coils, both electronic modules, Replaced the carburetors (were rebalanced as well).
Some people told us that this problem is present because this engine does not have a slip clutch, I personally do not see that as a cause. ( I did not get a valid explanation of that diagnostic and many 912 are running without the clutch and do not have this problem)
Possible cause (my assessment, and I am not a Rotax Guru by any means): When the engine is started with the throttle retarded (normal start) ignition is provided initially by the pick-ups at 4 Degrees BTDC then, (at a given RPM I supposed), ignition is provided by another set of pick up advancing ignition to 26 degrees BTDC. It seems that it does not jump to the 26 degrees and since it is apparently stuck there, the engine runs rough and vibrates heavily.
However when we start with the throttle advanced it does appear to jump to the 26 d and runs normal.
Is there a possible remedy, repair or adjustment that could be done to the pick-up unit or just replacement will be necessary (providing this is the problem) WE checked the Pick-up impedance according to the engine manual and that as well seemed within normal range.
Now I do not have any detailed explanation on how this “jump” occurs or how it works so if someone could help me I would be very thankful. As well, if someone has any other suggestions they will be very welcome.