Last year I Bought a new Webra 52AAR for My helicopter. After one gallon of running (PowerMaster 15%) , the piston came black and the retaining pin of the ring make a spot on the top of the piston.

I think it was to lean or the Nitro was to high. I heard that Webra need to run with 5% or 10% of Nitro or else I need to put a Shim on the head.

So I rebuild everything and I put a shim on the head. But how many shim I need to run this engine properly with 15% Nitro?

Do you have any advice?

Thanks?
Serge