Kostas,


...Only 20% nitro, or more is needed.

Also, as little as possible Castor oil, but no less than 20% total oil.

This engine will likely overheat with 15% nitro, or with less oil.


A 30% heli fuel mix is even better.


This goes for ALL YS engines, not only the .53FZ.
Only the 1.40 Sport can do with only 15% nitro.


But you might also have another problem, from what you are describing.