Losing coolant at manifold heat riser - Why?

If it is filling with coolant by itself, you have a crack. There is no coolant in the intake in taht area. It's only for exh gasses to reach under the carb for faster warmup. There is coolant in the head casting near that passage, down near where it starts from teh exh bowl. Are these ported heads? It may be that it's cracked, but only leaks when hot (like when you're getting on the throttle, as opposed to idling). A magnaflux check is the only way to make sure on iron heads.