Engine 'run-on' !?!

bit o' help, if you don't mind chaps.
came home from bvf at the weekend, and the engine in the bus was driving fine, nice and smooth etc, but it was getting bloody hot, and its started 'running on' when i turn the ignition off.
I checked the timing and it was at 10deg. so i retarded it back a touch to 7.5deg.
its still drives nice, no pinking or flat spots, .... but is still 'running on'.
only difference i can tell is its now running a bit cooler.
how do i stop it from running on ??

what carbs you running

it's got a single progressive.

running ons normally timing aint it ?

I've got the timing setup spot on .... and it drives bootiful !
just got this annoying 'run-on' when i turn ignition off ....
never had it before, and it's just annoying !

i dont really no ., just going on things if herd over the years . does the plug gaps sometime make it do the ?

and also what does the solenoid on the side of the carb do (if it has one ) could that be stuck open ?

got elec ignition, so don't reckon its that.
don't think the carb has soleniod ?
at the mo i'm thinking that its defo a fuel problem, 'cos type 1 aircooleds don't have fuel cut-offs like modern stuff (feel free to correct me, and call me a nob , if i'm wrong on that!). and if you jab the accelerator when you turn the engine off, effectively flooding it, it don't run-on.
maybe it's the fuel/air mixture ... but the carb ain't like a standard pict 32/34 etc, so I dunno where to (or even if you can) adjust the fuel/air mix !?!