BerlinaBob here: -as one of your respondents stated, -your V8 is running-on likely owing to carbon/build-up within the combustion-chambers, --which larger particles glow-red and ignite (engine shut-off) any residual fuel-vapor being induced owing to the slight opening of the carburetor-butterflys at their idle-position. Although low-octane petrol in a very hot-running engine (try running a 180*-thermostat instead of 195*; and dumping the fanclutch in favor of a flexfan) is particularly prone to this condition you are experiencing, other obvious first things to check for are: to rich of fuel-mixture (idle-screws settings? carb./float-level to high? carb./metering-jets to large? -or, worn intake/valve-seals allowing oil to be sucked into the combustion-chambers?) which would thus result in creating excessive chamber/carbon-deposits. Some Cadillac-dealers have provided the service of blasting minute walnut-shell particles into the chambers via the sparkplug-holes, and suctioning-out the refuse (leaving the chambers factory-fresh, -while harming nothing). Another "old Mechanic's-trick" (for a fee of $100) is to (without customer seeing), --simply trickle (from your garden-hose) water into the primary(where the choke-butterflys are) Carburetor-venturis for about 5-min while the engine is running at about 2,500-rpm (the engine will slow down as you increase the flow of water, so if you kill the motor pull the hose away IMMEDIATELY, -as you don't want excessive water to create hydraulic-lock by the piston's compression-stroke within the combustion-chamber, which can create serious damage as unvaporized water/liquid will not compress like air/steam). The trickling-water into a necessarily "Prewarmed-engine" is instantly turned into really hot Steam (noticeable at your tailpipe), --which acts to readily soften and flush-out the caked-on carbon-deposits (out your exhaust system), --thus you will find the engine runs more smoothy and without dieseling thereafter. Start with just a tiny spray of water from the hose-nozzle, and increase to a flowing-trickle as you become more confident(brave) with the procedure... :eek Another way of combating the dieseling-problem you are encountering, is to install a simple 12v.dc/carburetor-solenoid so as to plunge-out and impinge against your throttle-arm during normal running, thereby maintaining your throttle/carb.-butterflys at a normal slightly-open (just a crack) position during idle; -but when you switch the engine off, the anti-dieseling/Solenoid is likewise instantly deactivated so as to thereby let the butterflys snap entirely closed (via your existing throttle-return spring), ---thus defeating the dieseling(run-on) because the engine is thus entirely prevented from breathing (no air/fuel-mixture can pass in via the fully closed butterflys). Many later cars employed this technique from the factory, -as a way of addressing/defeating "dieseling"-complaints by concerned drivers like yourself, who sense that that violent/dieseling-action can indeed be harmful to an engine's pistons & conrod/wrist-pins...:s
