Carlos,
When you start the car in the morning or when the engine is dead cold do you have to keep moving the throttle a little so the accelerator pumps keep the engine running or does it sit and idle OK once it starts without touching the throttle? If it stays running all right then the idle is too rich. This is probably why your plugs are black.
Al, I know why the smaller venturi will work better, higher air velocity is the idea. I don't know much about Webers, I'm a Holley guy so I'm wondering if I can get rid of that stumble by playing with the accelerator pumps? If this won't work then at some time I could switch the venturis out. I changed the main jets from 155 to 145 and it made a huge difference in mileage, from about 10mpg to about 20 but it's still not where it should be. Sorry I hope I didn't hijack your thread Carlos.