I have a 1970 383 4pd Road Runner; no ballast resister or Mopar ECU; running relatively new MSD distributor, coil, spark plugs and wires. Also, relatively new Quick Fuel HR-680-VS HR-Series Carburetor 680CFM VS. Car starts right up in the morning and runs with no issue. Can turn off and restart right off with no issue. If I let sit for an hour or two, the engine just cranks with occasional cylinder ignition but won't run. Have checked for spark and voltage at coil and all seem fine. Confirmed fuel getting to carburetor and tried starting under hood with solenoid and manually closing choke, still no joy. Car ran fine up until three days ago and since then continues to repeat these symptoms. Any suggestions or thoughts on where to look next?

 

Check the spark when its cold and then when its acts up and report back?
I'm guessing its a Distributer or a Ignition Coil failing possibly?
What MSD Stuff did you install?

 

Spark is good both cold and warm. Believe I have narrowed it down to a vapor lock issue. Tested it again today, started right up this morning (8:30am) with no issues, repeated at 11am, repeated at 2pm; at 4pm it took about 20 seconds of cranking to start; at 605pm too about 35 seconds of cranking to start. Once started it ran funny for about 45 seconds and then cleaned right up. Running an MSD 8386 Ready to Run distro and MSD 8202 coil. Has mechanical fuel pump and no return vapor fuel line.

 

Sounds like your on to something.... Insolate the fuel hose from heat. And also Using a carb Insulator base gasket will help that issue.
Also look to see if the carb is boiling over or dripping down the throat after warm? is so, you may need to lower the carb float level just a tad.