UPDATE:
As you can see from the pic below, the XS650 timing advance spring is not the same as the XS400 spring.. This is the timing advancer from my 79.
These are not precise measurements although I did use my digital caliper. I was hard to get ID lengths because of the angle of the spring hooks and spring stretch, but they are close enough to tell the difference...
Keep in mind that the XS650 springs are NOS and never used. The XS400 springs have an indicated 16,000 miles on them.
The XS400 spring measures at .7" or somewhere between 17.8 and 18mm, this is the ID of the end hooks. The coil diameter is .2395" or a little over 6mm.
The XS650 spring is, .8", or roughly 20.5mm at ID end hooks. Coil diameter measures at .22" or 5.62mm..
The 650 springs use a thicker coil wire and they seem like they are stronger, which would mean that the advance would be slower when installed on a 400. This is probably due to the weights on the 650s advance are probably heavier as well..
Even though the springs for the 650 are longer, they seem like they will mate up to the 400 advance. I may try to use them and see what happens. Maybe I can use a 400 spring on one side and the 650 on the other side..
Well, I just installed a 650 spring on the advance.. It is MUCH harder to swing the weights to full advance. Im not sure that it would even hit full advance at 7k rpm..
XS400 on the left, XS650 on the right:
