My son's '68 Dart has a 600 cfm vac sec Holley recently installed on an Edelbrock Performer manifold on a 318. Engine is otherwise stock, but has HEI ignition upgrade. Gas mileage is dismal at 8-9 mpg in town stop & go driving. Inspection of the plugs would suggest that we're running more or less OK ratio-wise, as plugs are off-white to light tan. Compression is 150 psig across the board. Timing is 18 deg BTDC with no vacuum advance. Primary jets are #66's, with a stock secondary metering plate. Total advance is approx. 38-40 deg. Idle vacuum is a rock-steady 20 inches Hg. As we can't seem to find anything else to suspect, I wonder if the primary jetting is off. The car runs fine, just the mileage sucks.......
Any suggestions....???
Thanks,
Mark.