Could be a lot of things. 1.How far are the headers from your mechanical fuel pump? 2.Do you have a carb spacer? 3.Did you fill up the tank before the summer blend was available? 4.What's your fuel pressure? 5.Is your tank vented?
I found that the gas started boiling at my mech. fuel pump because of the header. I used an infared temp gun from Autozone for about 29.00 bucks. Just some thoughts.