It is strange!
To confirm - you have a good spark at each plug and plugs are getting wet when you're cranking the engine over. That would suggest fuelling and ignition is OK.
It would suggest a timing issue? Can you get a rough idea of timing using the crank pulley timing marks? Static timing should give it enough to get it to start.
I wonder if it might be a problem with the dizzy shaft? Maybe the worm drive has sheared a tooth and she's skipped a few degrees.
"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10
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