Random engine rattle, loss of power

[JP1975]

Just bought a 1975 xs650 that has sat for awhile. cleaned the carbs & points, put fresh gas and battery in it and it started fine, ran for a few minutes then made this terrible rattle like it dropped a valve into the cylinder when I'd rev it, lost tons of power but went away at idle, then after a minute it completely went away and ran perfect again. Shut the bike off and noticed I had forgot to plug the vaccum port for the Petcock. I thought maybe it was just really lean & detonating from getting hot? Plugged the port, started it up and no noise.
Rode it to a bike event today about 30 miles round trip, it road awsome! No noises at all, then as I'm almost home cruising about 55mph all of a sudden it get the death rattle again like a rod is about to come through the case, total loss of power, I could barely keep it running, this lasted for about 30 seconds, I slowed and made a left hand turn onto my street, sound stops and full power is back, runs perfect again. I'm completely confused, it does make a very faint valve / timing chain ticking but this noise is very loud, can clearly hear it wearing an open face helmet at 55mph over wind noise and the exhaust noise.

 

[JP1975]

Also should add I believe it to be from the left cylinder, which when I got the bike had dirty points For the left cyl and wouldn't spark on that cyl, I cleaned them out real quick with fine grade sand paper on the bike, got fine spark and seemed ok?
Only thing I can think of is possibly the advancer is somehow getting stuck advanced at high rpm causing detonation and loss of power, then vibrates unstuck and everything goes back to normal?
It only happened after the engine has been at high rpm, like reving in nuetral or high speed cruising, it won't just idle and do it. That's what's making me lean toward the advancer? Does that sound reasonable

 

[TwoManyXS1Bs]

Welcome to the forum, JP1975.

Fingers crossed, it's possible that the left side ignition is intermittant, drops out occasionally, you lose power, and that side clatters.

Best I can recommend is a full tune-up, new points, condensers, plugs, oil, ...etc.
And, with the tank off, clean and recondition all the bullet type connectors around the coils...