1970 Pontiac Firebird starts hard when warm

My Pontiac 400 starts hard only when it is hot. The
engine is stock with a .060 overbore and a pertronix
ignition. It also has a heat shield on the starter solenoid. I have also insulated the fuel line to the
carb. thiinking the fuel was boiling off. None of these
mods made any difference. Any suggestions?

Thank You