Thanks for the suggestions.
The aircraft is a Aeronca Chief with a Continental C90 Engine and MA3-SPA carburetor. It is on a PFA Permit to Fly.
As far as I know the mixture control does work progessively, but never having tried it in the air or on the ground I don't know how linear it's operation is. I'll definitely try it on the ground first.
I'm keen to use it correctly as this should extend the safe range of the aircraft which is not generous when two up.
If it works on the ground, I'll try it in the air when within gliding distance of an airfield, and when in practice with PFLs. I am taking comfort in the fact that at least two people reckon the engine is unlikely to stop in the air even with the mixture down to ICO.
Thanks again