I have a Picco (Sonic) 1.2 Stock engine, which confuses me.

I have used this engine for several races in 2016. Performance was ok, but it was hard to tune (inconsisten idlding etc).
I have started in engine in 2017, but since then is was running much too rich.
I have to turn the HSN from 4turns nearly to 1 turn and now it is running ok.

But I am really confused and worried what should have change so significant.

Really would like to here your thoughts about that.

Cheers,
Florian