My Mto Sport 912 is due for annual during ground run we noticed RPM drop for a while when exceeding 4500 RPM then returns back to normal !
Any owner has same issue , looking for solution please .
You likely should check fuel filter (clean or change it) and also take the carb bowls off and empty them and put them back (align the sealing gasket into the groove carefully). If it still persists, you have to take out the carb jets and clean them because they probably have a particle lodged into them. If you have a Rotax mechanic or an old motorcycle mechanic who knows carbs in general that would be helpful. You should download the IPS for Rotax 912ULS that shows exploded view diagram of Bing 64 carb for the mechanic to look at
If all that is good, then other internal issues could be the cause
Somebody at our airport recently had an RPM drop that was the same as the drop when preflight testing the ignition - we've suggested he start looking at plugs, leads etc, but it could get expensive. No idea what's going on with it now.
I'm with Abid - Small leak in carb or bowl gasket allowing air leakage is only evident at higher RPM's. Friend had the same issue and took him forever to troubleshoot - had a small warp in one of the carb bowls that only was evident at ~4000 RPM. He replaced the entire carb to verify and fixed the issue instantly.