Holley Carb Jetting Help, Lousy Fuel Mileage

Reddartowner said
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.

Hook up the vacuum advance(one that is not leaking), mileage will improve. I have never had a mild small block with a 600 cfm Holley perform very well with #66 jets. I run the 600 on two small block chevy's and my Duster 360. Try jetting up two sizes, you can run more advance without pinging and may actually pick up mileage, definitely pick up power. Holley's are a mid fifties design, they were not designed for economy.

For a base line, I run a 600 on my 360 with an edelbrock performer, headers and 3:55 gear with 2400 stall. It has #69's for jets. Car runs on regular without pinging, has a performance vacuum can with adustable spring, hooked up to ported vacuum. Car gets 12-14 in town, throttle response is impressive.