@DB90, did you manage to sort the no spark issue? I have a client's low mileage and very OE 1980 that ran a treat for weeks whilst in the shop waiting on chrome bits to arrive, and I rode it ≈10mil of a Friday and come the following Monday wouldn't rev beyond 1300ish RPM, loaded or otherwise. I've checked and found the following to be in working order or within spec:
-dynamic compression
-fuel jetting
-checked carb balance
-battery voltage
-disconnected the rectifier and ran it
-ran it off a car battery to insure it has the proper voltage
-measured the voltage directly at the TCI box replaced the coil with a new
-the voltage and grounds directly at the TCI connector (6-pin, btw) & other grounds in system
-magnet in rotor has what appears to be adequate magnetism
-fuel flow through the petcock
-to make certain ran it off a test tank with 93 & 100 octane to remove the possibility of fuel delivery issues
-clean the carb passages & jets
-reset float height & verified needle & seats function properly
-airflow into the airbox
-removed kill switch from ignitor circuit
-new spark plugs, 3 pairs of NGK just to be certain in recommended heat range and a step up and down, just to be certain
-Verified connections from pickup coil to TCI box
-verified connections from TCI to coil
-changed plug leads & caps(getting desperate)
-verified tightness on igniton coil & pickup coil fasteners
-removed pickup coil cover to eliminate potential grounding
I just bought an oscilloscope (which I feel like a chimp trying to land the space shuttle whilst using it) and've yet to hook it up to measure the pickup output whilst running at idle against the TCI output, and also as it increases in RPM. That'll be next unless one of the old timers here has a 'Hey, stupid...' to throw my way. I've been thorough, but am ostensibly human historically prone to err.
I found at least a few other threads on this forum where XS owners have had the 'it ran great [insert single-digit number of day] ago but now only idles' Covid variant, all with bikes running the electronic ignition system, but the threads all seemed to've petered out or went the, now closed, Pamco route. If I had a second known-good TCI box, I'd toss it in. I've ordered one but it has yet to arrive. Any links, insight, advice, or motivating insults are welcome. I'm verging on talking him into converting to points (ignition seppuku) or going with the match, petrol, whoompf! option.