I am impressed and greatly appreciative of your insights. This weekend I ran the experiment which you suggested. I dropped the float a whole lot...about 1/2" or so, and the fuel level was noticeably below the bottom of the sight glass. The motor ran much leaner, in fact so lean that I was worried. At 2K RPM, I was seeing AFR in the high 15's to low 16's at light throttle. At 3K highway speed the AFR was richer but still in the 14's. The really lean mixture worried me some, so I switched the IABs from .072 back to .070, and this helped; I didn't see excursions of AFR into the 16's anymore.
The question is "why did help so much". Check out the attached image. The original fuel level is at the bottom of the top piece of tape (mid-sight gauge). Just as you suggested, the fuel level was right at the bottom of the first e-bleed. Given the motor vibration and road quality surface, it is easy to imagine that the fuel was at times covering and uncovering this bleed hole. This is likely the cause for the AFR swings.
The very low float level is marked by the top of the bottom piece of tape. This would have put the fuel level a little above the 3rd e-bleed. Note that the jets you see in the picture are blanks.
But now there is still a mystery. I was not happy with the float as low as it was, and decided to raise it up some. I moved the adjuster 1.5 flats, and that brought the fuel level right to the edge of the bottom of the sight glass. This had a very noticeable effect on the AFR, with 2K cruise now in the low to upper 14's. So it changed the AFR by at least 1 point. Highway speed is now solidly in the mid-13's. The mystery is why did it have such a big effect? It looks like the 3rd bleed would have been covered with the very low float, and now the fuel level should be right in the middle of the blank e-bleed. Perhaps with fuel slosh, the very low float setting was allowing some bleed air in???
As I mentioned before the factory setup had the top 2 e-bleed open (drilled to .028), with a blank in the third, and the fourth open. Having the upper 2 allow some bleed air now seems reasonable, but I feel that 2 .028 bleeds is too much. At the moment, I am considering drilling the second bleed to allow some air in, perhaps something like .020.
In my next post, I will show the comparison of the QF 4 e-bleed metering block with an old Holley one.
