Carb Tuning - Richer? Leaner?

[protour73]

Exactly when is lean, too lean without engine damage? I know rich is always the better default but at what cost to fuel economy.

I'm in the process of fine tuning my carb (Pro Systems 750cfm double pumper), and at cruise (55mph, 5th gear, 2000rpm) the Wideband O2 sensor is showing about 15-15.8. With just the slightest touch of the gas pedal, it goes to 13.7-14.4. I'm 15-17"s of vacuum at idle with a 6.5 power valve, thinking I should be at an 8.5.

The lean condition was a bit worse, mid 16 on the gauge before I jumped from a 74 jet to a 76 on the primary. Throttle response is really good and it gets into the secondaries nicely.