I got some numbers here.  I thought I'd calculate the velocity through the venturi and then the available pressure drop at the fuel orifice.

20,000 rev/minute
.061 cubic inches
.01 square inch passage area  (Ap)
.038 sqare inch at venturi entry (Ae)

Volume = .061 x 20,000 = 1220 cubic inches/minute  (assuming total efficiency as a pump...probably noticeably less in reality)
Velocity (at passage) Vp = 1220 / .010 = 122000 inches/minute x 1/12 ft/in x 1/60 minute/second = 169 ft/second

Similarly, Veloctity (at entry) = Ve = 44.2 ft/second

Benoulli tells us that pressure drop = 1/2 x .00238 x Ve^2 x ( [Ae/Ap]^2 -1)

This calculates out to .21 psi.  That isn't much to draw in the fuel!  No wonder muffler pressure and crank pressure make such a difference.

There is more to it than that, of course.  The obstruction creates a pressure drop, which is why a spraybar with the hole at the bottom still works.  I need to do a bit more research to figure that one out.

On 45' lines at 3.8 second laps, I calculate that ram air pressure is about .05 psi.  That is a 25% improvement over the suction I calculated above.  Point those tank vents into the wind, dudes.   

One fix for the varying pressure is to solder up the vent, and drill a very small hole.  The pressure can then change only very slowly, slower than a half lap time, so the internal pressure is more constant.  This could be a restrictor in a piece of fuel hose slipped over the vent, so that fueling is not slowed down.