These spark plug pumps actually had little if any petrol mixture in the delivered air - the device that screwed into the spark plug hole had a valve that allowed air to enter from the atmosphere on the intake stroke - with the throttle closed and idling there would be a fairly high vacuum in the intake manifold, so atmospheric pressure would ensure that the majority, if not all, of what entered the cylinder to come via this valve, rather than the inlet valve. I have seen two types of valve, one is a rubber diaphragm about 30mm across that covers a circle of 6mm holes on compression, the other is a slot in the side of the body, with a spring steel reed valve a bit like what is seen on some two strokes.
The major criticisms of them are that the air brought in is unfiltered, if you have an alloy head you really don't want to do a lot of pulling plugs in and out, and finally - by the time you have aired up four tyres, the device is often hot enough to start melting the hose - careful when you touch it to unscrew it!

