Topic: Low Voltage On Charging System - 1978 CB750F

[cowboycletus]

Hey Guy,

Long time listener first time caller here. I’m the proud owner of a new to me 1978 CB750F. The bike runs decently and I’ve been joy riding while I do all the routine maintenance/find the little things that need fixing.

I suspect my charging system isn’t working properly. I replaced the battery last week. The one that came with it looked like a museum piece. Before I replaced it though it did leave me stranded once and I had to take it to AAP to have it charged. I’ve also completely drained the new battery and got it back yesterday.  I left it to sit overnight and made sure the battery didn’t lose any voltage overnight.

Today I’ve done the standard 1000-5000rpm/measure voltage test. The results were:

Battery alone
12.9

Battery in key off
13.0/13.1

Key on
12.3

Idle
12.5

2k RPM
12.7

3k RPM
12.9

4K RPM
13.0

5k RPM
13.0

Adjusting the set screw on the regulator makes no change. After resetting the point gap and core gap my max voltage is 13.5v at the battery terminal at 5000rpm.

I apologize for the redundant nature of this thread. I’m new here and this my first vintage bike.

Thanks for your help!

[bryanj]

For a start 12.9 is not a fully charged battery.
Then try disconnecting the headlamp and testing again, the US  idea of Nader to have lights on all the time kills batteries and does not leave a lot of generator output to charge the battery.
You need to be driving round at about 5000 rpm to consistently charge the battery

[scottly]

For a start 12.9 is not a fully charged battery.

[scottly]

Cletus, with the key on, engine not running, measure the voltage from the positive battery terminal to the black regulator terminal. This is the voltage "drop" through the harness to the regulator.

[cowboycletus]

I tested the “voltage drop” and it measures the same from battery to regulator as it does across the battery terminals, indicating no drop or loss of voltage.

I went for a long-ish ride yesterday, making sure to ride a little harder and keep the RPMs up, hoping that the fault with the charging system was a result of me taking it easy and not understanding the nature of the bike. After returning home and two electric starts trying to navigate the narrow alley behind my row home, the battery voltage measured the same as when I left.

Any ideas on how much voltage is lost on an electric start? Is it a reasonable thought the voltage was higher/the battery was more charged before those two starts at the end of my ride? Assuming the battery has sufficient charge to start the bike, but is not fully charged, will routine/daily riding recharge the battery the rest of the way or only maintain the voltage it started with?

Thanks for the help!

[scottly]

The voltage drop measurement is from the POSITIVE battery terminal to the black regulator terminal; if you measured  the same 12.xx volts as from the + and - terminals, you did it wrong.
The starter draws 100 amps or so, so certainly can discharge the battery, and it takes time to replace that power. What was the voltage before and after? Real numbers would help.

[cowboycletus]

My bad scottly, I was measuring with the key OFF.

Voltage Drop (measuring from POSITIVE battery terminal to BLACK regulator terminal with key on/engine off)
0.8v

Battery voltage KEY ON
12.3v

Battery voltage KEY OFF
12.6v

This morning my KEY OFF measurement was 12.9v. I don’t know if that loss is indicative of anything.

Thanks!

[scottly]

.8 volt is not bad, not great, but not bad. You need to take measurements running again, starting with a charged battery, and preferably kick starting the bike so you don't have to wait for the charging system to replace the billions of electrons the starter ate.

[cowboycletus]

Copy that scottly.

After riding around a bit today the charging system seems to be doing what’s it’s supposed to but the battery won’t charge past 12.9v while in the bike. I carried my meter with me and checked voltage every time I stopped. I also only used the kickstarter today.

I’ll put the battery on the trickle and make sure it’s charged to capacity and then do the voltage/rpm test again. I’ll report back with my results.

[scottly]

Is that 12.9v when the engine is shut off? If so, what is the voltage with the engine running?

[cowboycletus]

Yes. That is 12.9 with engine off and key off.

At idle it measures 12.6