Dieseling or engine run-on is a condition that can occur in spark plug, gasoline powered internal combustion engines, whereby the engine keeps running for a short period after being turned off, due to fuel igniting without a spark.
Dieseling is so-named because it is similar in appearance to how diesel engines operate: by firing without a spark. The ignition source in a diesel is the compression of the fuel in the cylinder, rather than a spark as in gasoline engines. The dieseling phenomenon occurs not because the compression ratio is sufficient to cause auto-ignition of the fuel, but a hot spot inside the cylinder starts combustion. An automobile engine that is dieseling will typically sputter, then gradually stop.
It's either starved for fuel, or there is an air leak. Are the crank seals, carb boots, and all gaskets in good shape? If you're running a boost bottle, chances are, it's a bad carb boot. If all of this is good, then pull the carb on the hot side, clean it, and be sure to remove the pilot, and main jet. Clean them really good, then replace them.