I have been using one for a long time also. Trailer hubs, exhaust header tubes, event the floor of my Comet to see if the Hush Mat insulation did it’s job. Everyone should have one.
Jim
I reread your original post. If it runs fine at highway speed airflow could to be the problem.
Or the fan isn't doing it's job. I have a jegs brand dual electric fan on a truck with a 302 & it wont stay cool at slow speeds or idling. I plan to put the stock fan back on eventually ,I dont use the truck much but it's aggravating like it is. it does fine on highway.
Fred and John have a point. Seems to be fine at highway speeds but has issues at slow speeds. Airflow would definitely be something to look at. 2100 cfm isn't really a lot. What diameter is the electric fan you have on the car? How is the fan wired? Run all the time? Work of a thermo switch? I think the factory fan for a 289 w/air was 16"dia 5 blade, and they have decent size blades.
Also could be a fuel problem or a little of both (airflow and fuel issue)
You posted you have a electric pump, where is it mounted? How are the fuel lines routed? My Comet had the fuel line run on the outside of the drivers side front frame rail, then up front by the sway arm mount it goes under the frame rail to the fuel pump. This keeps the fuel line away from heat. If your fuel line is run on the inside of the frame rail, inside the engine compartment the heat from exhaust/headers and such could be messing with it. If the electric fuel pump is mounted inside the engine compartment that could also be a problem. Seen electric fuel pumps mounted under car that have problems when they get hot they quit working till they cool down. Inside the engine compartment have even more problems, not a good place for a electric fuel pump. When the car shuts down have you checked to see if fuel still squirts in carb? Check for spark when it shuts down?
My 70 Torino is doing the same thing. My original radiator was leaking so I replaced it with a big block Torino aluminum radiator. The car has a 351 Cleveland in it the only thing that changed was the bigger radiator and I lost the fan shroud. I know the problem is simply the fan shroud the minute the car slows down and starts crawling the temperature and starts climbing right up in the car when it gets to a certain point as close to overheating the car will start stumbling in wanna kill. I would seriously check your airflow and water flow
Only use the brakes to stop and get gas.
Projects
65 Comet 421Stroker
93 Mark VIII Supercharged w/Fogger
70 Torino GT