The diagram shown above shows the metering rod resting on the bottom of the casting while the diaphragm is sitting above the bottom of the vacuum chamber. And that isn't quite correct. If the diaphragm is up in the chamber, the thinner part of the metering rod ought to be inside the jet and not on the bottom of the casting. The adjustment is made like this: Press on the top of the diaphragm shaft until you feel it bottom out in the vacuum chamber (or when the clip stops travel by hitting the top of the bonnet). Then push down gently on the metering rod. It should bottom out on the casting at the same time the diaphragm rod bottoms out. You'll get the feel for this. If it bottoms out first or if it doesn't bottom out soon enough, bend the lip on the metering rod arm to adjust it. If this isn't clear, please advise. The key is: on the bottom of the diaphragm there will always be manifold vacuum when the engine is running. If that vacuum is strong enough to overcome the pump diaphragm spring (the large spring), then the diaphragm will be sucked down in the chamber. It may be sucked all the way down or it may only be sucked part of the way down...depends on the amount of vacuum present and also on the mechanical limitation (more on that in a moment). The adjustment procedure assumes you have idling vacuum and that will be strong enough to suck the diaphragm all the way down. At this point the thick part of the metering rod will be inside the jet...giving you a more lean mixture and the rod will have bottomed out on the casting...that's the point of the adjustment. Getting both to hit bottom at the same time. Then when vacuum drops (engine under load), the spring will pull the diaphragm up and the thinner part of the metering rod will be drawn into the jet making the mixture more rich. This can and will happen without any mechanical movement of the accelerator pedal. Or if you mechanically move it...by pressing on the gas pedal slowly enough not to make for a lower vacuum situation but just to slowly increase speed while keeping vacuum constant, then the metering rod will be mechanically drawn upward...also enriching the mixture because you'll need that to maintain speed. So in the high-speed circuit, the vacuum might be enough to suck that diaphragm down, but the linkage might be keeping everything up. And in the low-speed circuit, you might be lumbering along at 20 mph, come to a hill but never ever change the accelerator pedal and still have the power circuit kick in when your engine vacuum drops to the point where the large spring takes control of your diaphragm/metering rod. This will simultaneously squirt some raw fuel into your throat, too. The upper spring is sort of an absorber on deceleration and also on acceleration it acts to delay things a bit.
