Rotax 914 loss of power during takeoff/flight

Bing carbs neumatically balanced?  Check for blue stains of 100ll as could be carb bowls are leaking at max map.

When the rpm drops, does the engine get rough or does it continue to run smoothly?

Stephen Kean wrote:

Bing carbs neumatically balanced?  Check for blue stains of 100ll as could be carb bowls are leaking at max map.

Carbs were last balanced about 30 engine hours ago. No stains noticed around the carb bowls.

Ken Ryan wrote:

When the rpm drops, does the engine get rough or does it continue to run smoothly?

I did not notice any engine roughness with the RPM drop. Also didn't feel any roughness on the ground during runup, or after flight during cool down.

Just some thoughts.

You don’t mention the aircraft type, is it tightly cowled, is an intercooler installed?

I see you are in Florida, was it a hot day & did you have extended ground running - long taxi, holding for traffic, etc before take-off? The 914 Turbo Control Unit (TCU) has a function that will automatically reduce power by opening the waste gate if the air box temperature reaches a certain figure (72C or 88C depending on engine/TCU serial/part number). There will be no other indication of this function occurring (no lights flashing, etc). Unless the manufacturer of your aircraft has installed an additional sensor and readout of air box temperature, you will not know if the air box temperature (as sensed by the TCU) is approaching or has reached its intervention temperature.

Heat soak in the cast turbocharger during your circuit may account for the second occurrence.

Installing an air box temperature sensor with cockpit readout would be handy I think.

Service Letter SL-914-014 has some information that may be of interest to you.

https://rotax.my.salesforcesites.com/DocumentsSearch/sfc/servlet.shepherd/

version/download/0681H00000FTeDYQA1?asPdf=false

Here is some more reading material, note the air box temperature mentioned in this document is 72C, in later engines this increased to 88C.

https://www.rotax-owner.com/pdf/UNDERSTANDING%20THE%20914%20ROTAX.pdf

The 914 Operators Manual describes a technique for setting take-off power then temporarily deactivating the TCU motor but that removes the temperature and other protections.

If the power reduction you describe is due to the above, I would suggest your engine is operating as designed, you could try using the Rotax TLR software to look at the TCUs history and/or MAYBE, very cautiously, fly again in cooler temps as a test.

https://www.rotax-owner.com/en/videos-topmenu/elearning-videos/659

Hope this helps.