Alex has a twin carb 1.7.
I do too, and currently I'm getting around 10l/100km, or 450km (280 miles) to 45 litres. I'm beating you Alex!
What kind of driving are you doing, lots of sitting in traffic?
What engine and exhaust mods do you have?
Have you tried reinstalling the standard airbox and cold air intake?
Over the years of owning my 33 I've noticed the fuel economy can vary considerably depending on how in tune the engine is. When I bought the car and it was running like rubbish, it got 350km to a tank. Some garage replaced sparkplugs, leads, air filter, distributor cap and rotor and the carb floats and did a tuneup including balancing the carbs properly, and the improvement was massive. We're not dealing with fuel injection which (so I gather, I'm yet to own an efi car) either runs perfect or reverts to a failsafe open-loop fuel-rich mapping if it loses a sensor. There's shades of grey, skill and magic involved in old Italian cars with twin carbs!
There might be some problem with your carbs (air leaks at the throttle shaft bearings, worn needle seats or somesuch carb tech I don't understand) causing incorrect fuelling.