Topic: Starting and/charging issue

[Paulius]

Hi everyone! I picked up a '75 CB750F a couple weeks ago and rode it home. Coming off a zx6r this was a pleasant change. The bike is in great shape but needs some work to pass safety. I've been working towards that. Yesterday got the tires changed out, overhauled the brakes, and fixed the improperly closing choke. After putting everything back together I took her out for a short spin around the block and noticed that one of the blinkers was out. Today I picked up a replacement bulb, replaced it, and turned on the power to test. Everything was going well until the gap between flashes got longer until it completely stopped blinking and stayed illuminated. Tried starting the bike, made some vague attempt and didn't even turn over.

Up until today it has been starting without any issues...up until today... So I began to troubleshoot. This is what I've learned:
FYI the battery is 2 weeks old
When everything is switched off the battery is above 12V
When power is switched on (engine not running) the reading across battery nodes was 8V and dropping
Turned the power off and the voltage jumped up to 11V and continued to rise
At this point the electric starter didn't even make a noise!
Thank god for the kick starter! Absolutely love it
After charging the battery and leaving the bike alone for a couple hours I was able to get it started with kick start. Measured voltage... Hovered around 8.5V... From reading numerous posts I understand it should be way higher than that!

So with no luck I turn to the experts. How can I troubleshoot? What's drawing all this voltage?

[calj737]

You really wanted to post this topic in SOHC BIKES, not in the Open Forum. 

If your battery is 2 weeks old, and it has suffered as you described, you may already have cooked that dude.
This is an excellent Charging System diagram to help you diagnose the parts and pieces before jumping into a replace this/replace that approach.
http://www.electrosport.com/media/pdf/fault-finding-diagram.pdf

What's causing the drain? Could be a non-stock headlight off far too high a wattage. Could be a very poor ground connection from your battery to the frame/motor. Are there aftermarket electronics installed? Accessory charging ports? Could also be... So best not to guess, but to diagnose.

For background, your bike does not make surplus voltage when idling or running the motor below ~2,500. That is why you need to sample the voltages at key OFF, key ON, at idle, then at a range of RPMs to know whether the system is charging, and properly. If you allowed the system to over-charge the battery (too high) you would also damage the battery.

You might copy this info, or simply start a new thread in the correct section when you get through your diagnosis if you're still struggling where more people will look for technical issues on new threads.

Welcome!

[Steve F]

When you rev the engine, does the voltage rise or stay flat?  Charging on these bikes won't happen below say...2000 rpm., but check that out first.
Being that you may have points, with the engine not running, you may have a set of points that were closed at the time you were checking your turn signal operation.  If that were the case, you would be energizing a coil all that time, and causing a significant draw on the electrical.  Try to use the kick starter to gently rotate the engine while checking the voltage to see if there are rises and drops in the voltage as you go around.
Systematically pull fuses to see if there is a significant draw on one of the circuits and start checking things one at a time.
You may have to check the contact points inside the voltage regulator for corrosion or burned out points.  They sometimes can be fixed if you're careful.

[333]

When you say new battery, did it come from a dealer?  And did they give it a good charge?  I've seen too many dealers just fill the battery with acid, and hand it to the customer. When I worked at a Honda dealer, we charged it for an hour on a heavy duty charger. Not a good thing to do on a regular basis, but it is a good thing to do for a battery's first charge.

Or worse, getting it mail order, and filling it with water (because they don't ship acid).