The leather diaphragm should hold 500 mm/H2O of vacuum with a leak-down rate of no more than 2 mm/sec. It should be flexible enough to collapse of its own weight when held by the rim. Nissan dealers can supply the special diaphragm oil required to soften the leather.
A screw under the circular end cover reacts with the auxiliary idle spring (3) to set the idle speed. Tightening the screw moves the rack to the right to increase fuel delivery and idle rpm. But the effect extends to all engine speeds (Fig. 5-33). Fuel delivery for best idle over-fuels the engine at high speed and, conversely, a setting that gives best fuel economy at speed, costs low-end power. The adjustment is a compromise.
Other adjustments require a test fixture available to dealer mechanics. For example, the stroke adjustment, shown at 4 in Fig. 5-32, is made to factory specifications with the diaphragm chamber evacuated and the pump turning 500 rpm. To further complicate matters, balance-spring adjustment must be done by comparing its action with a known good unit.