What you have here, is unburnt fuel entering a hot exhaust. The exhaust itself can be hot enough to ignite the mixture, so there's no need to wonder where the spark is coming from.

The old SUs did not have an idle cut-off solenoid. Shutting off the ignition does not shut off fuel flow. What stops the fuel from flowing, is the stopping of the engine. When you cut the ignition, the fuel keeps mixing with the air in the carbs as normal for however long it takes that engine to spin to a stop.

So, I've got to go with the idle speed theory. An engine that's idling too fast will take longer to stop after you cut the ignition, and so it will pump more air-fuel mixture into the exhaust.

Other tuning factors can aggravate this by providing, say, a hotter-than-normal exhaust or an excessive amount of unburnt fuel to begin with. So, as always, be sure to check your basic tuning stuff.

Most of the time we just sort of shrug and accept it when the engine won't idle all the way down to spec... alas, in this case I think you might be forced to actually do something about it. Finding root causes, fixing air leaks and then going back and fixing the newly-upset mixture, all that stuff.

Good luck with it!