Do you have a vacuum gauge to hook up to the intake manifold? This will help to explain what's happening while the engine is running I have a large vacuum gauge on the dash so I can monitor the engine all the time.
At idle (throttle plate closed) the engine is sucking air but the throttle plates aren't allowing air in so there's high vacuum in the intake manifold(assuming you don't have too radical of a cam). As you open the throttle, the primary plates let in more air and the vacuum lowers. The power valve starts to dump extra fuel when the vacuum reaches the rating of the power valve. in tour case, 3.5 and 4.5 in/hg(vacuum). The more load the engine is under the lower the vacuum reading will be for any given throttle opening. By putting in a higher vacuum power valve, the sooner the PV will open which corresponds to lower throttle opening or lower load that the engine feels. You want to adjust/change the vacuum spring to correspond to the PV opening.
But remember, I work on boats and a boat is under ever increasing load as the throttle is opened. It's like towing a trailer uphill in 4th gear. Also a boat engine has to be run richer because of the load(to reduce the chance of detonation)