What fuel pump is running the Weber? They hate high fuel pressure (no more than 3-3.5psi) And what is the base idle like? If it's a little on the high side it will encourage the engine to try and 'run on' after the ignition is killed. Is the coil in good shape? A weak coil will break down under load which will result in loss of power and an increase in fuel usage. What oil are you using? Might need to factor in a heavier weight oil (especially in summer) and is it a mechanical head or hydraulic? It may only need adjusting if it's a mechanical head*.

*ignore the book on this one. With the engine warmed up to full operating temperature, adjust each of the lash adjusters by feel. Just a little rock when the lash adjusters are off the valves. Get it right even with feeler gauges and it'll sound like a typewriter...