Howdy and welcome pau 13z and a howdy to Norm and Speedie.
I remember reading about the phenomenon of a spark being apparent with the plug out, but not being strong enough to sustain under compression. Is the spark a strong blue or a wierd colour that doesn't come on strong? Try swapping the ignition coil if all else has failed perhaps.

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!