Voltage Fluctuations at High RPMs

[nicola]

Hey all, had my '77 360 running last weekend, only been doing short rides with it, 2-4mi, while I work out the last little kinks. Though I had it pretty good until I rode it about 10mi and it started dying at low RPMs. I figured it was an electrical issue and after charging the battery, it started right up again. I replaced the battery last year with an AGM that I found with close specs, my bike is kick-start only. That being said, the bike did sit for most of the year while I fixed other parts of it. So the battery might be bad, but before I replace it again, I want to make sure the rest of the charging system is working.

Following the shop manual, I tested voltage across the battery at 2k RPM and get around 14v or so. Moving on to the field coil, I measured 4.2 Ohms. On the stator I measured 0.6 Ohm between the yellow wire and each of the white wires, and 0.9 Ohms from whites to whites. The spec said 0.72 +-10% so it's a little off, but consistent so might still be fine. The diodes on the rectifier seem to be working fine, although they were completely reversed with what the manual said, perhaps my meter measures diodes backwards?
Edit: There was some extra resistance through the meter's leads, after accounting for that the stator measured 0.7 Ohms white-white. Also checked from white-ground to check for shorts, everything looking good there.

Now here's where I am confused. I am testing the regulator as the manual says, start engine, rev to 2k RPM, disconnect battery positive, measure voltage from fuse box to ground. The voltage seems to drop as the revs increase... Below about 4k I get 13-15v fluctuating, and anything higher than that drops the voltage to a steady 13.3v. This sound to me like either the stator is not putting out enough voltage, or the regulator is not working correctly. It's the old style regulator that's apparently adjustable, not quite sure if adjusting it would help since 13.3 seems way low for it. I can tell somethings up because all the lights fluctuate with RPM.

One other thing I checked was to see it there was any current running through the system when the key was off. I disconnected the battery and put ammeter between it, tested for current with both kickstand on concrete and on rubber, no flow either way. Don't know why there would be but motorcycle guys always tell me that can kill your battery.

Just wondering what to check next, and if anyone has any thoughts on this? I'm going to take my battery in to get a proper load test on it. I don't mind replacing the regulator and rectifier with a modern combined unit. Just trying to rule things out before replacing them. Thanks in advance

Edit: Took battery into shop, load test said bad battery. Still wondering about charging system though. Also any recommendations on sealed batteries? The 2D only needs a 7Ah according to the manual

Edit 2: Tested output from stator while running, approx 78v AC between poles at around 3k RPM. Pretty sure stator is good.

 

[Wolfe_11B]

13.3 is low but it will still charge the battery. Probably has some shitty old wire and bad connectors.

 

[BBS360]

Believe it or not at first glance it looks like everything is working properly, for the factory setup. Unfortunately your SLA/AGM battery is not stock.
The gurus may correct me but since the voltage regulator adjusts charge rate based on battery voltage I don't think the disconnected battery test will give you any meaningful results over the specified rpm for the test.

Since you're not running the stock battery you may have overcharged your battery, killing it. Your 15v reading is likely too high for the battery. Probably OK if you never Rev too high too long, but apparently it was too much.
I'd either adjust the stock regulator so it doesn't exceed the duty voltage of your battery or get an aftermarket electronic regulator.

 

[nicola]

Probably has some shitty old wire and bad connectors.

Actually rebuilt the entire harness and replaced all the connectors last month...

I'd either adjust the stock regulator so it doesn't exceed the duty voltage of your battery or get an aftermarket electronic regulator.

Sounds like it's probably the way to go, seems most new regulators are integrated with the rectifier, should just have to put the right connectors on the right wires right? I've noticed a lot of people on here just use some reg/rec from auto store, recommendations? Think I might just get a replacement battery of the regular kind, though it would be cool to go sealed but seems like it's been more hassle than maintaining a normal battery.

 

[Wolfe_11B]

I've used a tiny sealed battery before, made for alarm backup, and it worked fine for a long time.

 

[BBS360]

I know drewpy has commented about his bike overcharging unless he keeps the load high enough on the bike. It works for him.

There's the basic non-adjustable auto store regulators. There are threads on them. They're cheap and appear to work.

I have discovered two other adjustable regulators that look interesting. Still need to do some research on the cheaper on to make sure it'd be compatible.
$18: http://www.mgcycle.com/index.php?main_page=product_info&cPath=37_161&products_id=1110
$55: http://www.oregonmotorcycleparts.com/vregulators.html