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Are you using anti-freeze? If so this could be the problem. Years ago when I got my 39, I had the same problem. I put a glass tube in one of the top hoses and could see the coolant foaming. The faster I ran the engine, the more it would foam which would then spill out the overflow. Switched to water and have not had an overflow in 20+ plus years. Can't explain it, but it has worked for me.