After trying all the simpler things I had the stock 26 radiator recored on my 68 GTX. When that didn't help, the water pump was replaced/upgraded just to be sure...as was the fan clutch. None of that gave any improvement, nor did a full carb restoration. But as mentioned above I would place the car in neutral and idle up around 1500-1800 rpm and that would help bring the temp down following an abrupt stop in hot weather. So I guess from what was mentioned earlier that indicated a flow problem (or an excess heat generation issue). Somehow the engine was generating more heat than could be removed at hot idle. It's possible the fuel/air mixture was not optimized though I tried an assortment of jets, rods and springs over several weeks. The exhaust pipe residue was always a bit sootier than I would have liked. Though no carb changes ever improved it. Also tried different types of engine oils to improve the heat removal. Idle engine vacuum on the car was slightly lower than I would have liked...8"-10" as I recall rather than say 14" That was 15 yrs ago and it still bugs me to this day it was never resolved. Though I do wish I had tried an Infra-red gun back then to remove all doubt. No one ever mentioned it.

The car also had an off-idle stumble at around 900-1200 rpm that I worked on at the same. So anything that could improve the spark and fuel/air mix got toyed with (air filters, coils, spark plugs and wires, carb changes, accel pump settings etc.). That never improved any so I figured the lower vacuum was the cause and wasn't going away. No vacuum leaks were found. Is it possible the slightly lower vacuum was a symptom of the overheating issue as well? Even with these 2 annoying issues I had a blast driving that car for 3 yrs. Almost still miss it.

RR69s