Originally Posted by
OddDuck
It's sounding more like a shop job, if the bike's worth that.
I'd guess it's likeliest to be an old battery which has finally died. They really don't like being left for a year (assuming that nobody has been tendering it for you). I'm not 100% sure if the battery is essential on this bike but it'll certainly help when starting.
You mention a strong spark when plug grounded... out of the engine isn't the same as inside it. You need more HT voltage to create a spark when it's dealing with a compressed mixture - have a look for Paschen's Law, this deals with this phenomenon.
The second possibility is the coil, assuming it's still the one held together with a cable tie (you don't mention replacing this). Cable ties relax a bit over time and it might not be tight enough any more. The coil might also have degraded further. On that note, HT leads don't last forever and if the insulation on the lead has broken down then you'll lose spark voltage.
Third possibility: reg-reg got hit with a voltage spike when the original stator died and isn't working properly any more. A total loss system (I.e. running off battery charge only) will still work just fine while the battery holds up but as soon as the battery is flat, that's it, no go. You should see 12 V static on a healthy battery with the bike not running, 11 V is the bottom limit before permanent battery damage with the ignition switched on, and 13-ish to a maximum of 14.5 with the engine running and revving, if the charging system is still working OK.
Fourth possibility: bad ignition switch, bad grounds to frame, or rotten wiring in the loom, particularly the crimp joint connectors to battery and coil. The bike's from 1982, that's nearly 40 years, and I've recently found that even crimp joints relax and corrode internally given enough time. If the copper conductors leading into the crimp are brown with oxide then the joint's suspect, if the copper has gone green then that's it, it's stuffed. Cutting back wiring and replacing terminals is possible but gets very fiddly very fast, if a replacement loom can be purchased at an acceptable price then that's usually a better way to go.
So... (just my guesses) could be battery, could be coil, could be rec-reg, could be loom/switch/grounds, could be any or all of them. As Honest Andy has mentioned, being sure that the carb jets are clear will also help, blast through with compressed air will be good but don't use wire, too easy to drill / file them bigger.
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