Throttle plates are open enough to allow air/fuel mixture to enter the engine as it "winds" down.
A hot (operating temp) engine will self ignite the mixture. Air/fuel contamination takes place during this time in the plenum of the manifold to you have to crank it a bit longer on the restart.
If possible, adjust the idle down as to get the plate opening a bit smaller. If the cam doesn't allow a lower idle, an idle solenoid should be installed for base idle and close the plates when ignition is turned off.