Engine run-on is after the ignition is switched off, used to make me laugh when my cars did this. I'd wave the keys at my friends with the engine going brum, brum, buuuuum, brum, brum, pop! Sparks aren't needed for run-on, there's enough hot spots in the cylinder to promote ignition, probably hot carbon I think. Been a few years now since I've had a SU in bits, used to tune these carbs and turn down the needles if I couldn't get a correct one. Setting up a very modified A series turbo was fun with boost at 12 psi! Had to turn it down to 10 psi as the needle was just so thin at the higher boost, almost didn't need a needle at full throttle and the piston raised!