Here's one, I've got a 68 350 Skylark.
Was running really bad so I rebuilt the carb, put in a new fuel pump and added an in line fuel filter. The ONLY time it's hard starting is after running and sitting for about 30-60 mins, (not 20mins, not 90mins-thats all good.) What i've determined is that the carb bowl goes dry, because after 40mins I take off the air filter and look down the carb, pump the throtle, I get 2 squirts of gas then no more squirts. The bolw is empty, i've opened the carb and checked. To start it in this 40min period, I hold the pedal to the floor and crank and crank it for way to long 60secs. and she starts up the same way every time, lean staring. I took the carb apart and double checked the float levels, all is well. i have a new radiator and running cool, so its not boiling the gas out of the bowl.

What do you think?

What type carb.? 2bbl or 4bbl ?

Sounds like vapor lock (when an air bubble in the fuel line, usually caused by high temps, keeps fuel from the carb). Not too common this time of year though. The timing of the problem is right though.

Thats all I got. Its happened to me before. Changing the thermostat has helped.

David

its a 2 brl rochester (02870140) is the model #.

Sounds like the fuel bowl is leaking down . You may have a well plug that is leaking or a crack in the bowl. You need to R&R the carb. remove the throttle plate body and fill the bowl and look for leaks ..also if your float level is too high and the throttle screw is holding the idle too high .. gas will be drawn from the bowl when car is cut off untill bowl is empty. HTH Guyopel:beers:

Guyopal- thanks for thinking for me! the idle screw is good. what your saying would give me a very rich start and a quick die and most likely a gas smell. I don't get any of that, it's just dry...2 pumps of gas then dry/ i figure that's the fuel in the pump galley.

do you think the new fuel pump could cause a vacume and draw the fuel from the bowl. (but the inlet to the bowl is on the top where the needle sits.) i think maybe banging my head on the air horn might cause things to change.

i'm going to clamp off the fuel line between the pump and carb to see if that indcates anything.

Thanks!