Vapor Lock - Fuel or Spark?

A heat-related no-start condition can be caused by either no-fuel or no-spark and both can resolve themselves once the engine cools down.  Make sure that you're fixing the right problem.  Electronic ignition modules and ignition coils generate heat and can overheat in hot underhood conditions.  Fixed dwell systems (like points & condenser, Mopar Electronic Ignition, Ford DuraSpark, etc) have coil saturation currents that are highest at idle and decrease with increasing RPM.  Variable dwell systems like (GM's HEI) use electronic circuitry to only supply enough current to saturate the coil and their coil saturation currents increase with increasing RPM. An HEI system uses 5.5 amps to saturate the coil and, at low RPM, this can generate around 17 Watts of heat in the module.  See HEI Versus Standard Ignition & Customising A Reluctor-HEI System To A TR4A & Fighting Vapor Lock.

The quickest way to check if you have a vapor lock problem is to see if there is any fuel in the carburetor.  Take the air cleaner lid off and pump the throttle a few times.  If you don't see a good stream shooting out of the accelerator pump nozzles, then you likely have a vapor lock problem.

If you have fuel with a no-start condition, there is a possibility that the engine has flooded due to the greater rate of vaporized gasoline entering the carburetor or being produced in the carburetor.  Flooded engines may be cleared of excess fuel by opening the throttle valves to wide open (full throttle) while cranking.  Never pump the gas pedal while trying to clear a flooded engine.  Once started, you can rev the engine slightly (enough to get the engine to run smoother).  If it can idle without stalling, start driving again.

To verify that you have a good spark, you can pull one of the spark plugs wires off of the spark plug.  Either install a spark tester or hold the spark plug terminal a 1/4" to 3/8" away from the plug.  While your helper cranks over the engine, look for a spark but be very careful not to get shocked.  Also make sure that the air cleaner assembly is installed on the carburetor to minimize any possibility of the spark igniting any venting gasoline fumes.

If you've determined that the problem is fuel related, continue on to the next page.  Otherwise, you need to figure out whether the coil or ignition module has overheated.  It is important to keep your ignition module cool, which was sometimes a problem with some HEI systems.  HEI modules are often installed inside of large-cap distributors and dissipate the HEI module's heat into the distributor body.  Make sure that there is adequate thermal paste between the HEI module and its mounting surface.  Moving the HEI module from inside the distributor to a cooler location under hood and preferably on a heat sink will also help.