ORIGINAL: ajcoholic
I forgot to add - one trick I have used when I figure an engine it running too hot - try taking a blowgun attachment for an air compressor and aiming it right at the head (make sure not to influence the air into the carb) to force cool it. You can tell immediately if the engine cools off some and starts to run better. If it is an issue of inadequate cooling, you can always build an electric fan!
AJC
I picked the picture who Micropuller had send to RCU about making own cylinderhead. You can see there are not enough cooling fins and not enough thinner cooling fins. A thick cylinderhead without or few cooling fins are good to save hot from combustion and difficult to reducing hot from the cylinder head.
In beautiful weather with sun can give warm asphalt who make warm air, there are not easy to keep engine cold.
Also increase thinner and cooling fins..
Jens Eirik