The SU/Hitachi gets the suction to raise the pistons via a hole on the outside flange, the one where you mount the filters. Not unknown for the holes the get obstructed by a gasket or the actual filter. This could mean that the pistons aren't rising enough, meaning the butterflies open and give you more air than the piston position wants, hence lean. How does the engine rev out in the lower gears? Does it have the same problem? Do the pistons rise fully if you give the engine a second or two's full throttle while looking down the carb throats? Loosen the dampers to check if the pistons rise smoothly and easily, using your finger.