Okay, here's the story: I recently did some work on the engine that involved draining most of the coolant. Upon filling with fresh coolant, I began by adding what I thought was about half the total volume with antifreeze concentrate; I would then top off the other half with water. (I now realize why it's better to mix 50/50 prior to adding to the car) To my surprise, the radiator was full way before I expected, and before I was able to add any water. The overflow reservoir was empty, so I filled that with water in hopes to at least dilute the coolant somewhat. I didn't do any other work to the cooling system other than addressing a couple very minor leaks. Now when I take it out, it runs into the 220-230 range within 25 minutes. Could the more concentrated coolant mixture be the cause for running hot like this? The car in question is a 71 Scamp, 318/904 with headers. 26" copper core radiator with a fan clutch.