Is my MPG way off?

[eschaider]

RS,

Your question is similar to one someone else (not using Webers) asked a day or so ago. This is a tuning problem! Get a pair of wideband sensors, one for each engine bank and one of the many handheld data logging gizmos available today. You will be able to make some pretty good driveability and fuel economy adjustments to the Webers.

As luck would have it, Engine Masters did an excellent session on tuning carbs. Admittedly, the Webers use slightly different names for some of the same circuits and probably some circuits the Holleys don't have. All that stuff notwithstanding, the principles are the same and translate fairly well across to your Webers.

The Engine masters session you want to search out on the Motortrend Channel or other channels with the same series is Season 7, Episode #6. They use an LS3 with a Holley 950 XP as their test bed to go about smoothing out the AFR across the engine operating range. Their approach is quite good, easy to understand, and, most importantly, pretty easy to duplicate.

The Engine Masters gang used an engine dyno. You don't need to. If you have access to a chassis dyno, that would be nice but not necessary. You can do the same job in the car, especially if you bring along an extra set of hands and use up a little bit of brake pad material.