The D400 is a little different and many techs don't like it and are not very good at adjusting them.
The lever adjustment screw should have about 2 threads showing when you start the adjustment process. This leaves the lever very loose but keeps it from interfering with the poppet adjustment.
The cap over the poppet assembly/spring should be about 1/2 turn below flush.
The Venturi adjustment is normally set at one click away from full open.
Leave the diaphragm and purge cover off for the initial adjustment.
Once the air is on check to ensure the lever is still loose so it does not interfere with the valve.
Increase spring tension by screwing the cap down (clockwise) 1/4 turn at a time until the leak stops. Screwing the cap clockwise can sometimes cause the spring to wind up a bit so you should adjust by going clockwise a little farther than needed and then backing up counterclockwise.
Once you get in the ball park, you can adjust back and forth to make the final adjustments to get peak performance. These adjustments will be very small with maybe only 1/8-1/12 turn in either direction.
On most second stages, you need to go about 1/12 tunr past optimum to anticipate the formation of a seating groove. With the D400 the spring pressure is a lot lighter and this is not required. (and if a slight freeflow does develop you can tighten things slightly later)
You always want to cycle the valve several times between adjustments to ensure the valve is seating properly.
During this process make sure the lever remains loose as is not preventing the valve from closing.
Once the poppet adjustment is done, you tighten the nyloc lever adjustment screw until a slight leak starts and then the manual says you back it off just enough to get the leak to stop. In practice, just a bit more detuning is needed to prevent the valve from leaking once the diaphragm and purge cover are reistalled, but you want to make sure that you leave it no looser than neccesary or the reduced lever travel will reduce the flow rate of the second stage. It should begin to flow with only slight pressure on the purge cover, if not, the lever is too loose.
There should be 4 small raised dots on the edge of the diaphragm and these should be aligned at top/bottom/left/right on the case to ensure the little guide for the lever is straight up and down and not angled.
In my experience, the plastic orifice in the later D400 can be a real pain in terms of getting maximum performance. In addition, the newest version of the poppet has a raised ridge toward the inside that often does not seat terribly well on the plastic orifice. A little gentle pressure to press things together is sometimes needed but a lot of care is needed to ensure things are properly aligned so that the orifice is not damaged. And don't get carried away with the slight pressure thing - too much can damage the orifice.
Care is also needed to ensure the hard tip at the bottom of the poppet does not damage the relatively soft plastic orifice during assembly. You need to install it with it centered in the hole and not bouncing off the edge of the orifice.
Care is also needed when inserting the plastic orifice in the aspirator body and many techs screw the orifice up at that point, making a light cracking effort impossible to achieve later on.
SP went with the plastic orifice to eliminate corrosion conerns (which were a non issue in the D400) and to reduce the cost of replacing a one piece $45 aspirator body/orifice with a two piece aspirator and $4.00 orifice if the orfice were ever damaged. It is now a lot cheaper to repair if the orifice is damaged in some way but the older one piece brass aspirator body/orifice was a lot better.
SP also changed to a slightly different lever late in the D400 production that has flats on two sides instead of one This was marketed as a means to increase flow rate, but the only change I have observed is that the vaccum needed to maintain the flow rate is increased at high flow rates once the valve is opened. It reduces freeflows, but makes the reg breathe harder.
