I have a 39 Plymouth truck with a 1954 218CU Plymouth flathead in it. Everything is pretty much stock. I have been fighting this exact same problem as New_castle for years with littte success. I Have new carb,fuel pump points, plugs condenser wires tuned up, etc. Have tinstalled all stock heat shields, spacer under carb, blocked off the heat riser and insulated the fuel line to fuel pump, added a few other shields around the bowl carb. I will look into some of the other recommendations here. I hope I find the magic solution here as I am at my wits end with this and I love my truck.