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Gen light /6 on at low revs/starting bike

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Andey
Posts: 117
Joined: Wed Sep 02, 2020 12:45 am

Gen light /6 on at low revs/starting bike

Post by Andey »

Hi folks,

This has probably been answered in some way before, but I couldn’t find a direct answer as it’s not quite broken.

I with recent riding something has started happening, I have the gen light come on when I’ve just started the bike and at low revs…

Disappears after 1400rpm but it’s back until the bike is really warm (more than 1km of riding)

This didn’t happen before, so wondering what to look out for…

Battery seems to be charging ok after a good 3-6kms. I haven’t been on any shorter rides…

I did replace the spark plug leads with beru ones… previously I had /6 /7 wires on there, they worked but had all cracked up…

Any thoughts on leads? I cleaned all the rotor and stator up about a year ago and has been working fine since… wonder if the diode board connectors need cleaning?

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schrader7032
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Re: Gen light /6 on at low revs/starting bike

Post by schrader7032 »

First, be sure your battery is truly OK. Check battery connections and ground. Should have the battery load tested. At a minimum, what does the battery voltage read at idle and at say 3K RPM. At high RPM, you should read at or near 14v.

Snowbum has some extensive discussions about troubleshooting the alternator and voltage regulator:

https://bmwmotorcycletech.info/trbleshootALT.htm
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

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srankin
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Re: Gen light /6 on at low revs/starting bike

Post by srankin »

There is no use in reading Voltage at idle to determine if the charging system is working as at proper idle it is not charging and as such, the red light will be on. About 1500rpm or so the light should go out maybe 1000rpm. If it doesn't you have a problem.

It is normal for the charge light to come on at idle. LOL, that is if the idle is set properly.

No light on at any time also means there is a problem, a burned out bulb will not let the system work to charge. St.
Owner of a 84, R80RT and 78, R100RS

Andey
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Re: Gen light /6 on at low revs/starting bike

Post by Andey »

Thanks gents for the replies!

So I had the front cover off as it’s been about a year and a half since I was last in there. Thought I might as well have a look at the condition of things… everything’s held up well, but I should really change the 3color stator wire as it had brake fluid drip down it for goodness knows how long by the previous owner… it looks like it’s been in a swamp (but it’s completely dry)!

Anyways long story short, took the diode board off to have a look and clean the area around. Some oxidation of connectors, but nothing horrendous.

Put everything back together and starts up as previous… but the gen light is much more responsive to the revs… no dimming then on/off…

Forgot to mention! 12.3v at idle… 14.14v @around 3000rpm

I think I might need to set the idle again as mentioned… resync the carbs, because it does feel a little different since swapping the spark plug cables out…

something forgot to mention, I don’t need to be on the choke for as long as previously… before I was riding a good 50 meters with the choke on, before switching it off.

Now it generally prefers the choke off once it starts to tick over.

Interesting that you might be right with the gen light… that it’s suppose to go on at 1000-1100rpm

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srankin
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Re: Gen light /6 on at low revs/starting bike

Post by srankin »

The charging light comes on at turn of the ignition key, a good check to make sure the bulb has not blown out. After starting, it will be lit at idle or up to about 1000 or 1500 RPM, then it should go out.

Proper idle for an airhead is about 1000RPM or less, maybe 700RPM. At that RPM the light will stay on and the system will not be charging.

Not to be a jerk but the proper order of tune up if you have a rough running bike or a bike that won't idle is first make sure the valves are properly adjusted. Second, if you have points, they must be gapped properly then when gapped properly, timing must be set. Last carbs are adjusted.

Now the caveat to the tune up is many adjustments HAVE to be made while in idle. So before doing anything, the cables for the throttle must be set so the throttles are in the idle position. Same thing with the choke cables, they have to be adjusted so the chokes are off.

Surprisingly once carbs are synced for idle mixture, and cable pull off, they will stay adjusted for a long time changing mainly due to minor cable stretch over time and use. Valves and points on the other hand change faster and require more adjustment. St.
Owner of a 84, R80RT and 78, R100RS

Andey
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Re: Gen light /6 on at low revs/starting bike

Post by Andey »

I’m a little surprised nothing much has “moved” for about 18months… I thought I’d be resetting a few things especially the points gap.

I follow the steps for setting up the ignition and carbs religiously… I believe it’s the only way to have the bike working reliably. There’s no lazy steps. If so, why bother taking the time… lol

Hear too many stories of peoples bike aren’t running properly after mis-steps or lack of attention.

But agree with the steps that it starts with the proper foundation checks. Valve adjustment, throttle cable free play adjustment etc…

I resynced the carbs… seems like natural cable stretch, the cables were brand new installed 2 years ago. And maybe the weather being hotter… but seems to have solved it… running smooth again now.

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srankin
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Re: Gen light /6 on at low revs/starting bike

Post by srankin »

My two bikes run the worst on hot humid days and best on cool days. It is the density of the air and a bunch of related stuff that causes the change. Luckily Bings tend to self correct to a certain degree.

Yeah after two years I would assume the carb pull off would need to be adjusted due to some cable stretch.

Getting back to the valves, I find on my two when they start idling bad, looking at my blackboard to see when I last adjusted valves shows the current mileage right on for another adjustment.

LOL, I got rid of points in the one bike and have electronic ignition in the other as well. No problems there, no adjustments in a long time either do to changing timing chain or set up after rebuild. St.
Owner of a 84, R80RT and 78, R100RS

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