Did someone forget to mention fin cooling area. With 30degrees C ambient, you have to make sure there is enough air flow past the head. I am not sure if the Swallow is enclosed cowl or not. Also check for correct valve clearances. .05mm on cold. Plus ofcourse fuel flow as mentioned by several posts above.
I only fly YS engines (all darn Eric Henderson's fault, he introduced me to them, nothing else seems better).

PineBaron
PineBarons R/C Club South New Jersey.