Voltage too low at coil wire!

<<<<< The coil on my carbed 1300 X is producing a pale yellow spark,......>>>>>

Brian, I fully agree with Colin on this. Stick with a standard-type 12v coil (you may well find your present coil is OK, if you solve the following problems)

9 volts at the coil + wire (ign on, not cranking) is TOO LOW! That's probably the reason for the 'pale yellow spark'!
I have checked the voltage (same conditions) on my '78 1300, and have got 11.2 volts at the coil + wire.
Somewhere along the ign circuit, you are losing around 2.5 volts - could be the internal ign switch contacts (seems like the ign switches on our "beloved" X's are prone to high internal resistances as they get older).

I suggest running a temporary jumper wire from the 'hot' cable on the starter motor, up to the + connection on the coil (be careful of 'sparkies') and see if the coil performance improves. If it does, go searching for voltage loss somewhere along the switch-to-coil circuitry.

Have fun!!

cheers, Ian - NZ