Fairbanks-Morse Magneto help

There are videos on rebuilding FB mags on U tube. I just did my mag on a older Lincoln welder.....
Id make sure there is no oil inside mag, meaning a failed oil seal. You mention new points and condenser
As long as the cap and rotor are in good shape and the points are clean and properly gaped, my thoughts are a failing coil.......I had exactly that happen on mine. I replaced everything but the coil and yet I had a weak erratic spark
My coil looked ok, no cracking. I checked it with a ohm meter and the readings were not as indicated
I replaced it and that did the trick
Good luck