RE: Gws gearbox gear change
Pinions and spurs are matched so that the distance between motor shaft and drive shaft remains the same. If you change one but not the other you will have a mismatch (perhaps small in changing from B to D but a mismatch nonetheless). The bigger diameter the spur gear is, the smaller diameter the pinion HAS to be and vice versa.
If that's a GWS 300 gearbox: Your B ratio was 62/14 = 4.43. D ratio should be 66/10 = 6.60, but you have made it 66/14 = 4.71. That (bigger than it should be) 14T pinion will be meshing deeper, and perhaps binding in the (also bigger) 66T spur - and, of course, you haven't changed your ratio as much as you really wanted to!
Cheers, Phil