I'm a numpty on engines, especially injected ones but some thoughts. From the manual the inlet valve opens 36 BTDC and closes 60 ABDC. If you translate that onto a 720 degree 4 stroke rotation, 0 degrees is TDC, and add 270 degrees to piston 2 to get it to the same reference then - inlet valve 1 opens at 684 degrees and closes at 240 degrees - inlet valve 2 opens at 234 degrees and closes at 510 degrees. - both valves closed 510 - 684. I don't know if my 1 and 2 are the same as Norton's references? If so then it is beyond me to imagine how 2 is robbing 1? If air is being supplied via the IAC and the air line was too small you could imagine that 2 could be fighting a partial vacuum on the air supply?