think I finally got it after 6 month

[karl pa]

Last summer I was riding my 73 GT750 when it started backfiring and popping through the carbs. Thinking it was ignition I replaced the points, condensers and plugs. After this it worked fine for about 50 miles when it happened again, this time it was so bad I had to pull the left plug wire to get it home. Thinking I may have holed a piston, I did a compression test and found all cylinders around the same. Then I made a vacuum tester to check crank seals, I used a pvc pipe cap and threaded a hose adapter through it connected to a vacuum gauge, removed the carbs and placed the pipe cap in carb boot and cranked over, all three were the same. At this point I was drawing a blank so I pulled off the entire points plate and wire harness to change with a spare, when I noticed a small rub mark where the harness went through a clamp under the motor, apparently firing the plug at the wrong time. Since repairing this it hasn't acted up, but I wont take it far from home till I am sure.

[Craig380]

That could do it - if one of the wires to the points had rubbed through the insulation, it could be grounding intermittently. And every time it grounds, it will fire the spark plug ... or the point wire could be broken internally, causing an intermittent connection.

Hope you've got it sorted ready for the good weather

[karl pa]

.. or the point wire could be broken internally, causing an intermittent connection.

I never thought of it that way, if the wire is broke internally, and loses connection when points are closed it will fire the plug.
Since I had everything apart and put back together, it may be making contact now but show up again. Thanks for the input