That's got nothing to do with the head design. You need to make sure the carb isn't set too lean and that all of the cooling fins are clear and there is no mouse nests under the flywheel shroud. You need to be running the engine at full throttle when under load.
Another thing that has nothing to do with any of the above...if the 1/4" thick gasket between the carb and engine is not there, the carb might be absorbing more heat from the engine as a result.
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