Your're right, bad timing can cause an engine to overheat. It's not cause it's running in a rich condition (that would actually cool the motor). If it's too advanced, it will detonate (ping or knock) and can damage the head, piston or even the block.
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he couldnt tell me why the car heated up so much the first night we had it back
Haha, I remember this - happened twice to me. When you change the coolant, the cooling system system should be bled or it can get hotter than normal. For me it didn't actually boil over, but the gauge going up freaked me out. When you fill up the radiator, there is still some air in the engine or hoses. To bleed it, you open up a high spot in the system and run it until the air leaks out, meanwhile keeping the radiator topped up (do this when it's cold, of course). Or just let it cool back down and top it up again.
Many cars don't have this problem, but the 1200 does. Some cars like the old Honda Accord actually had a bleeder valve (just like for the brakes) in the cooling system. The 1200 doesn't.