Spark plug tire inflator

[Isambard]

If you look closely at the body of inflator you will see 4 slits, these opened the cylinder to atmosphere,
so on the intake stroke of the cylinder no vacuum was created in the manifold to draw the fuel mixture,
into that cylinder. Under pressure, the slits were closed off.
The inflator worked best at an idle, reving the engine appeared to overcome the valving.
As the Poms would say, bit of a chuffing success! :-)