<VV> Implausible carb issue?

[Grant]

I am working with someone who has a 140HP engine using 4 primary carbs. The carbs in designated primary position have relocated jets to the side for cut out prevention. All 4 have vent tubes. The problem seemed to be lack of fuel and no accelerator pump squirt on two of them. There is an inline electric pump at 3-4 lbs. Upon removing the tops, it was discovered that the two driver side carbs did not have enough fuel to reach the slot in the accelerator pump cavity (thus no squirt), but that the passenger side carbs were full. Had to be a plugged line, but found nothing wrong. A little trash in all of them but nothing to write home about. Venturies are dirty, indicating backfiring. Really want to try determine what is wrong before re-rebuilding them. No gremlins found. 
How could this be???
Thanks,
Grant