I just purchased a used 53 and new 63. Both run great. My issue is with both engines but because I am flying the .63 now this is the one I will discuss. I own OS, Magnums, Saitos but these are my first YS engines.

Facts:

Fuel - WildCat 20/20
"F" plug
Laterally mounted on a Funtana
APC 13X6 prop (broke my 13X4W)
Good smoke trail throughout throttle range.

This engine runs perfect at all throttle setting. Response from idle to full open is fair, but I believe could be better. Seems a bit slow to me. But maybe it is just new.

The only situation that concerns me is a rattle at 1/2 throttle. Almost like predetonation. Is this a charactoristic of YS or is it something else I need to change. I have less than a gallon through it. It may be there at full throttle, but I dont hear it much. I'll check the rpms tonight and let you know what that is.

Perhaps too much prop?

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I doubt that there is anything seriously wrong. It could just be a harmonic in the airframe at that throttle position. Your low to high transition can be adjusted with the regulator. It may be too rich if the transition loads up. Try going in 1/2 turn on the reg and see how it behaves.