Clois, retarding the timing should 1) make it easier to start not harder, 2) take away pinging (rattling the valves) not cause it, and 3) never blow back thru the carb.
All of the three things you mentioned are typically a result of too advanced timing, not too retarded timing.
Also, just as a test, when you get your new thermostat, put it in a pot of water on the stove. Turn the heat on and put a thermometer in the pot. Watch the temperature on the thermometer. As the temp reaches the thermostats rating, you should actually see it start to open. By the time the tempurature is 5 degrees above the rating, you should be able to see the thermostat fully opened in the pot of water. It is actually an interesting and fun experiment.
Next week we will start on mirrors and magnets.
Ed