I have noticed with my Nano that the motor tends to '4 stroke' at intermediate power settings, indicating the mixture is too rich.
I was tending to avoid those settings on descent/landing just in case...
The plugs looked fine, but the exhaust was throwing a bit of black stuff onto the rudder.
I decided to have a look at the carb settings.
The jet needle was set at the 3rd slot down, whereas the Hirth manual specifies first clip position.
That made sense- I suspect the Fusioncopter guys had purposely set the mixture on the rich side while the motor was bedding in.
I reset to the first (top) slot, did a ground run then a longer flight to check operation, temps and fuel burn.
The motor runs sweet across the throttle range- EGTs and CHTs well within the green, plugs light tan.
Fuel burn over a 1 hour cruise at 5000rpm 75km/hr was 15 litres.
I'm happy with that.
After another 10 hours flying I checked the plugs again.
They were on the light side of tan- not too bad but I like to see them browner than they were.
So I have set the needle clip on the second slot down- midway between Hirth (top slot) and FC (thrid slot).
I will check the plugs again after a 1-2 hour flight.
I think there is something you should know about this case.
The positions of the needles are changed in the factory in Warsaw. We do this due to seeing a bit too high egts in the 4400-4900 RPM range in our climate. Also for this reason, the engine ecu software has been modified for fuel injected versions.
There is a possibility that in other places than Poland other setting will be the best and safe but we recommend extreme caution when modifying carburetors adjustments. And we advise against it to people without experience in this matter.