Similar issue on our 914-based gyro. Just started recently. Engine would suddenly surge in flight (usually around 30" manifold pressure). Adjusted waste gate as per Rotax video and we haven't seen the in-flight surge since but now on full power, the engine will suddenly surge up to 40" and you need to cut the throttle quickly. Any ideas?
You should check or calibrate your throttle position. You may be operating in the nasty 100%-115% zone so the TCU thinks you want take off power and can be unstable if you do not move smoothly/quickly from 100% to 115%
Read "Understanding the 914"
http://www.rotax-owner.com/en/support-topmenu/technical-information/rotax-technical-articles/17-understanding-the-rotax-914
Thanks - we tried that. Adjusted the wastegate per the Rotax procedure and recalibrated the throttle. But it didn't help and anyway I don't think it's that, as when it surges, it goes over the normal 38" turbo max.
Sounds like you fixed the problem with the wastegate adjustment but now have a different issue. But what you describe is normal; you should go over 40" at full throttle (see overshoot note in the Operators manual P 2-2). After a few seconds it should settle back down to 40" (OK, 39.9 but who looks that close?)
slow increase in turbo could show a need to lubricate /clean the wast gate bearing. It would also be a factor in turbo over shoot and map instability.
Things that can lead to this problem include
bad floats
leak at the float bowl
low fuel pressure
leak in the vacuum lines
bad connection to one of the sensors
corrosion on a sensor connection
plugged map sensor
servo cable kinked or corroded ( too much drag)
bad ground to the engine
The throttle setting above 35 inches is very fast and not usually set to some power other than 40' so yes it will be very fast from 35 to 40. this is a normal condition.
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