First the basics: '86 Yamaha FZX700 Fazer, new battery. FWIW I am mechanically and electrically competent so I don't mind getting dirty and doing my own debugging work. Sorry in advance for being long winded but I have been kinda fighting this problem on and off for a while now (well I thought I fixed it but I guess not)
So early this season I had taken the bike out a few times and (after cleaning the carbs) she seemed to be running well. One day I took it to work and it just happened to be the first day of the summer that it was pushing 100 degrees out. At the end of the day I found my bike dead as a rock, battery reading 2.5 volts or somewhere around there. Got a jump and it ran fine on choke but if I let it idle down it would die (only running for a couple min at this point). I figured the battery just needed more time to charge than the 2 min I had given it before letting off choke so I got another jump and took off before letting it idle down, ran fine the entire 45 minute ride home (Though I did keep the revs up at stop lights for the most part, etc). Seemed to run & start fine for a few days after that indecent so I thought maybe my battery is getting a bit old and the heat somehow managed to drain it. Got a new battery and haven't had any problems since then, probably 2-3 months now.
Two weeks ago I lost my keys so the bike went unused since then. I pulled the ignition last wed and brought it to a locksmith on Thurs to get new keys made up. Got the ignition installed on thurs and the bike fired up fine. I finally found time to go for a ride yesterday. Ran errands all afternoon and I must have started it at least a dozen times with all the stops I made. No problems, started strong every time
Went out a few hours ago and today it was dead as a rock again. Got a jump, rode it around a bit (maybe 10 min), shut it off and it was dead again (only had enough power to slowly turn the engine over once).. Another quick ride after another jump and same deal. So I am kindof stumped what might be causing the problem. It never seems to have any problems while running. For the past couple months it has seemed to stall if it idled for more than a couple min with no choke but I think a quick carb cleaning fixed that since it didn't have any stalling issues yesterday.
Any ideas where to start looking? Currently all I can think of is one of 3 possible issues:
1) intermittent short somewhere. But I don't know if this is it... I would think that if there was an intermittent short that is able to drain the battery while the bike is switched off it would likely also be able to cause loss of power while riding, which has never happened.
2) bad relay somewhere. Maybe a relay isn't properly latching to the 'off' position when I shut down the bike... most of the time it shuts down OK and doesn't drain but other times one of the relays doesn't latch off and drains the battery. I suppose this could also explain why it runs OK when it is on but occasionally drains when shut down.
3) weak/bad stator. The stator is providing just enough juice to run the bike but is not charging the battery.
Only reason the stator is at the end of the list is because I feel like I should notice the battery getting progressively weaker over time if this was the problem, which it doesn't. It will start strong one day but then the next day it is completely dead.