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Besides Ron's suggestions and comments, an effective overflow container helps avoid rust and other contaminants in the cooling system by keeping air out. The overflow tube has to discharge under the surface of surplus coolant in the overflow container, so that reverse flow back into the radiator takes in coolant only. For this to work, the radiator cap has to seal against the top of the radiator fill pipe to maintain a partial vacuum in the overflow tube so return coolant flow up from the container will happen. And the cap has to have a lightly spring-loaded seal in it as a check-valve to let this reverse flow of coolant enter the radiator top tank when the engine cools down.

Check your spark timing and that any vacuum advance provision is working effectively. Late spark throws more heat into both cooling system and exhaust. Consider using more advance than specified if the original spec was based on fuel octane lower than what you are using.

C.O. Greenlaw, Sacramento Cal