Hi HUW,
I am one who has put this mixture control in the trash

, because the part in old age, can make mistakes

, and it does not effect.

You must for the intake, cylinder front let a rest Rubberhose (tupe or piepe) and close tightly even with a screw cap or that no air can be drawn.
To your problem .... you say that the close of the vergassr You misfire?
May be the carburetor, electric with too rich a mixture, or incorrect.
Even a low battery or defective TCI (everyone always says CDI) ... can do that.