ORIGINAL: Pull Up Now!
In other words, the dominant pulse comes from the crankcase, and the diaphragm reacts before the pulse can ''escape'' out the intake. And it's enough to scavenge the oil coming from the crankcase.
That's what I reckon
Definitely the spring on the pumping diaphragm makes no difference in this configuration. It 'works' either way. So much that just flipping the prop draws from 8" head, a 1/8" per flip unchoked.
In the interests of science I now have a WT37 and a WT38. If the 37 proves too small (I think it will by tacho readings) then I have other plans for it...