After getting my milage up to 23 mpg, I tinkered with things a bit to try and improve it. The idle was way too high and I knew one of the sealed butterfly vents was not fully sealed.

I tightened and re-adjusted my rear brakes (4-turns out from locked), removed, cleaned and re-sealed the vents in the throttle plates.

By all rights this should only improve things, but my mileage is suffering again. It is still better than the 16 mpgs I used to get, but not as good as the 23 mpg I got on the last tank. I'll need to gas up tomorrow and recalculate my miles to be exact, but I fear it is down around 19-20 mpg.

Moreover, the car is running a bit rougher as though it was too lean... But if anything, I've reduced or eliminated vacuum leakage, so I'm confused by this...

My question here is about the carbs. Can having them too lean make your fuel economy worse? I know it can damage the engine, but I'm not clear on how it would use more fuel per mile.