When you said you have a weak spark on the front, I assumed you had a good spark on the rear cylinder - my mistake.
If you have 12V going where its suppose to go, make sure the ground wires are good and cleanly attached to the frame. People always check the 12V wires and forget the return path back to the battery.
That spark tester is very interesting, I have never seen one before, now I want one.
Also check the ground wire to the battery from the frame, and check the voltage on the battery (at the ECM if you can) while its running.
Something that happens frequently on bikes with electronic ignition, when the battery voltage drops (old battery) the ECM does not have enough voltage to fire the plugs. It does not get weaker and weaker, it hits a lower threshold level and just shuts off.
Its possible the battery is right on the edge while running.
Last edited by KCW; 12-17-2018 at 08:14 PM.