You have made great progress.
Concerning the coolant temperature. Prior to doing anything else. Get a non contact thermometer and find out what the temperature actually is. I would take the temp at the thermostat housing, that should be close to the engine temperature sensor. The dash gauges of that era were not extremely accurate when new, add in 50+ years of use and it is surprising they work at all.
Chrysler did change the engine mounted sender over the years to one with a different ohm range, some one may have swapped in the wrong one.
But first, I would check to see how hot it really is.

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Doo Ron Ron and the Duke of Earl are friends of mine.....
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Nice truck, good you are making progress.

Usually these cars have previous owners that were not so careful about what they were doing. Good idea to check what you have before.

About the temperature, remove the thermostat elbow and remove and check the thermostat. Chrysler had thermostats for 195F, 180F and 165F . If you have a 195F (higher temperature is supposed to be more efficient), you might want to go to 180F and see how it behaves. In my car the 195 thermostat shows the temperature needle alarmingly high (even if it works ok), I moved to the 180.

About high idle, let us know what carburetor the car has now (remove the air filter, check stamped letters, send some pictures). After confirming what you have the manuals link recommended by SlantSixDan is very useful to set them. Check carb float height. My car also stumbled heavily until the engine RPM increased, and had a high idle. I had the chance to fix it buying an NOS (New old stock) original carb (very expensive today).

Valves should be set with engine hot and running. If your engine's idle is high, you probably better warm the engine and then shut it off and check one cylinder. Some oil droplets will splash when the engine is running and the valve cover removed.

Good luck, keep us posted.

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/6 Dodge Charger 69
...there's nothing like using the "Search" link above to solve most of your problems...

Hi, Thanks for the suggestions. When I first bought the truck and noticed the temp issues, I started by doing the easy stuff... thermostat and water pump. Going back to look at my packaging, the thermostat I put in IS a 195, so maybe that's all that's wrong now. I'll get another 180 and try that.

The carb is a BBS 4658S (according to the tag on it) with a manual choke. The screws do show signs of someone having it apart at some point, so once I get a gasket/rebuild kit, I'll check out the internals.

The high idle is not too bad. With my timing light, it's showing about 650-660 warm idle with the lights on. Can't get it any lower, though. Doesn't seem like I should have the idle screw out all the way and STILL have a high idle, but I can't find any vacuum leaks either, so... not sure what's up. If I firmly press on the throttle plate arm (not enough to bend it, but... firmly), I can get it to come down to more like 550, maybe there's carbon build up preventing the plate from closing all the way?

Well I figured out the off-idle stumble. I got a carb rebuild from QuadrajetParts.com and going through the carb, I realized the check ball at the bottom of the accelerator pump well was missing! Put the one from the kit in, and magically, no stumble!

Still fighting the high idle, but at least I'm getting closer!