I have a 1979 GMC c3500 2wd 1ton dually. It has a 454 with an Edelbrock 750cfm 4 barrel carb, Holly aluminum intake, Long tube headers and a stock distributor. My timing is set to 11 degrees. This just started randomly about 3 weeks ago, at an idle my truck will very slowly start to over-heat; meaning if I let it idle for 5-10min it will climb to over 220 degrees. While driving it runs fairly normal (175 degrees like it always has before). I have so far replaced the factory radiator with a new one from AutoZone, I have replaced the water pump, radiator cap (16psi), radiator fan (flex-fan), new fan shroud, I put in a Mr. Gaskets 180 degree Thermostat and I’m using an overflow with the new radiator. I’m not losing any fluids. I have a hard-line temp gauge and I have a temp gauge in my radiator cap so it is not my gauges. I have no anti-freeze in my motor oil and I have no motor oil in my anti-freeze. For the most part the truck runs good other than some times while accelerating the motor sounds like its popping and there is a loss of power. (I had an Edelbrock foam air filter and one time while going down the road it made some popping sounds while accelerating and I noticed that the foam air filter melted a little, so I changed the air filter). Normal day to day driving my trucks runs good. I need to know why my truck is over-heating at an idle. What checks can I do? I don’t know the mileage on the motor but seems to run healthy at an idle, and driving down the road for the most part.
Have you flushed the radiator? As far as the popping through the carb, when you say the timing is at 11 degrees, are you refering to BTDC? Also, have you checked compression to make sure you don't have a burnt intake valve?