Rebuilding those pumps will go a long ways to improving the flow. The diaphragms get stiff and the original pickup lines will soften/collapse and often develop pin holes and/or cracks. New diaphrams can bet found or cut from stock nitrile sheet. Tygon can be used for the pickup line. The pumps have a series of spring/ball checks. Soak in hot soapy water helps loosen things up. Blast with WD-40 through the channels.
When healthy, the pumps will spray a decent line of oil from a 20" bar.
I've a couple earlier ones that had the short-lived adjuster screw. It only functioned as a gate valve to limit the flow on the pump, not to increase.