For reasons I won't go into, I've got a Malpassi Fuel Regulator with 8mm pipes on it. If I were to use an 8mm --> 6mm reducer, would that act as a venturi and affect the fuel pressure?
(A venturi is used to create a low pressure by accelerating the flow of a fluid for suction or flow rate measurement) Only if you can empty your tank in a few minutes through your engine. A carburettor has a maybe 1.75 to 2.5 mm diameter fuel feed hole going in. Provided the hoses feeding it are larger by a bit, its not going to do anything really to restrict the flow of a few tens of litres per hour..or under .. 20 mpg at 60mph.. about 4ml per second to mix units. Or 1/7 oz .. I would worry more about increasing the number of pressurised fuel carrying joints. Unscrew the 8mm fittings and fit 5.5mm ones..
No. <edit> In theory it would make it speed up, but you've got the float chambers in the carbs buffering the fuel supply, so as long as the pressure doesn't overwhelm the float valves, everything will be the same.
Then use adaptors or change fittings at the other end if you can. But basically this is not worth worrying about relative to the protection the Malpassi will give against Weber needle valves leaking. Fit the Malpassi with adaptors and get on with other important things.. There are far worse crimes to commit elsewhere like timing and mixture errors.
