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I have been through some of what your are experiencing with my own carbuerated cars and those of other rodders. MOst of thetime it is vapor lock. It could also be the carb or timing. I have found what Tom K is suggesting is rigt on as to the fuel pump and the location of the regulator. I would add that the return line will help your car run better CONSISTENTLY, so even if ithis is not the exact problem, I would encourage you to put one in with a high end fuel regulator with an accurate fuel pressure gage installed.
Answering a few questions would help give you some suggestions.
Have you checked your fuel pressure? If so what is it?
Do you have a fuel regulator. Is it adjusted to the proper fuel pressure for your carb. Too much fuel flowing to the carb at start up is just as bad as too little.
How old is your electric fuel pump and what brand is it? Is it mounted correctly? Some units need to be mounted so that the unit is horiz vs vertical. Also they need to be mounted on the chasis rail so that the incoming fuel line is at a certain distance from the fuel tank and cannot have a point in it that is lower than the inlet of the electric fuel pump. If installed incorrectly they will cavitate for awhile causing a hard start problem. That's why it's a good idea to know what your fuel pressure is at the carb or fuel log at strat up and while running.
Have you noticed that when the car is off and sitting for a while, and you first open the fuel cap, that your hear a slight sucking sound for a split second, like there is a vacumn in the tank? We have found a couple of SPF cars that had a problem with the gas tank venting properly. It caused a vacumn in the tank especially on hot days and caused problems for the fuel pump.
What size is the fuel line?
Have your floats in your carb been adjusted to the proper height?
What is your vacumn at idle?
Have you adjusted the idle circuit lately?
What type of fuel does the area of Michigan have that you live in when you experince this problem? Oxygenated or not? We have found that some oxy fuels in Arizona, boil at 140F. That means that they can boil in the fuel lines and/or float bowls on a med warm day. Different parts of AZ have different oxy fuels and other parts don't have oxy fuels.
What type of fuel log are you using?
Have you checked ALL of the the fuel filters? Besides an inline unit, sometimes there is one in the fuel logs and the inlet of the carb.
What is your initial timing?
I know it is a lot to consider, but each one could influence your situation.
I would say knowing that your fuel pressure is correct is the first thing you should check out.
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