Topic: 75' CB750 lost all power

[lafindufondu]

My 1975 cb750 project is nearly complete but now it wont start.  All of the electrics were working after I replaced the left and right hand switches two weeks ago.  I went to go for a ride yesterday and turned the key, hit the start button and lost all power.  I'm pretty sure the starter motor didn't even engage.  I did noticed a spark out of the corner of my eye, it looked to be right around the #1 carb, when I hit the starter button.

My battery was at 11.7 volts (brand new motobatt) so I started charging it.  I turned the key with the charger connected and I had the neutral/oil, headlight, taillight all working again.  I figured it was just a weak battery so I let it fully charge.

After fully charging the battery, I turned the key and had lights but when I hit the starter I lost all power again.

I checked the fuses and noticed I wasn't getting any power to the 7 amp fuse, I checked the wires behind the holder.  The fuse still good. I go into the headlight bucket and see that the two Brown/Red with White/Yellow tag are not connected to anything. 

I had two Black/Brown wires from the left/right switches connected to black in the harness.  I connected these Black/Brown wires to the Brown/Red in the headlight bucket and am getting power to the 7 amp fuse (which I believe to be headlight) but I'm still not getting any lights.

I'm a little lost and have a lot to learn when it comes to electrical.

[lafindufondu]

Last night I tried bridging the red and black wires to the ignition switch and got nothing.

[lafindufondu]

I'm getting no power to the headlight bucket.  Black/white wire at coils is getting power with killswitch and off.

[TwoTired]

Use the wire diagram as a road map for power routing.
Use a voltmeter to follow power from source to the point where power on the map is lost.
Or, Use the map and voltmeter to trace from where power isn't back to where power is.

Both methods will tell you where to look for a mechanical break in the path where electrons cease to flow.

However, do check your fuse holder front clips and back solder connections.
Also, check your fuses electrically as well as visually.

Sometimes wiggling switches will expose and intermittent connection.

Cheers,

[lafindufondu]

Thank you

[lafindufondu]

I disconnected all wires in the headlight bucket.  I am getting power to the black wires from the harness.

When I probe on either of the black wire + the engine as ground I am only getting 10 volts.  Battery is at 12.5. 

EDIT: I connected only the right hand switch and hit the starter button and heard a click from what I am assuming was from the starter solenoid.  No lights, was just testing the starter button.  Only heard that one click and nothing after that.

[bryanj]

Either the battery ground connection is bad or the battery has failed

[TwoTired]

Measure the battery voltage when you engage the starter.  If it drops below 10 V under load, the battery has failed.

Cheers,

[my name is nobody]

And just to be sure, recheck your battery ground cable connection on the frame (in the
general area of #1 carb you mentioned?)....just throwin that out there

[lafindufondu]

I tightened up all of the motor mount bolts a few days ago, so I thought.  The bolt that the battery ground goes to was loose 

[HondaMan]

Remove the fuses from the fuseholder (disconnect the battery first, please) and, using a pencil or small dowel, wrap some fine sandpaper around it and spin it inside the fuse clips. If you have the OEM fuseholder, chances are near 100% that the clips are corroded now, because their original chromate plating was a 10-year-life product. They will likely have a dark brown color to them, instead of a shiny copper look.

I make a newer fuseholder for this bike, which uses ATC type fuses (available everywhere, and more durable than the old glass ones). PM me for details if you want one?

[lafindufondu]

Thanks for the tip Hondaman, I'm going to do that with the oem fuse holder for now.  I will be picking up the blade style fuse holder from you when I have some extra money... already on my list 

[rocket johnny]

 :)for  cleaning the oem fuse holder i use a  .22 cal brass cleaning brush .  cheap and works good

[HondaMan]

:)for  cleaning the oem fuse holder i use a  .22 cal brass cleaning brush .  cheap and works good

+1 !