1977 cb750 electrical problems

I just bought an old cb750 recently and now I'm having some electrical
problems. I was riding down the rode when all of a sudden the bike
stalled out. I pulled over to the side of the road and I noticed all
the electrical was out. I tried kick starting it and push starting it
and I even got someone to try and jump start the bike but all I got was
the power to flicker a couple of times. The battery is brand new so
that shouldn't be the problem. I don't have the money to go take it to
a shop and get reamed so I want to try and fix it myself. I think
maybe the power is shorting into the frame somewhere, but I don't know.
Anyone have any advice for me? thanks in advance.

-Joe

 

Try following through this fault finding chart. It helped me get through
the electrical issues with both the bikes I'm working on. (74 RD350 and
81 KE125)

cheers

 

sorry, this is the link to the chart.

http://www.electrosport.com/Images/fault_finding.pdf

 

Looks like I got some work ahead of me, but at least I have some ideas
of what to do now. Thanks alot guys.

 

Reluctant to state the bleedin' obvious, but have you checked the main
fuse and ascertained that you've done the battery terminals up tight?

 

Honda (and other manufacturers) may have done this to *protect* the
alternator from
overheating due to excesss voltage being generated during the daytime
when the lights didn't have to be on, or maybe they thought it would
save a little horsepower.

But what it tended to do was cut the alternator output in half whether
it was daytime or night time. That resulted in a lot of dead batteries
when it became mandatory to run the headlight during daylight hours.

The typical alternator has a 3-phase stator. Each phase generates about
80 or 90 volts by itself, but, since the electricity has to return to
another phase, the voltage across any two phases is 90 X 1.73 = 156
volts.

If any one of the three output phases has a bad connection, the voltage
output is dropped by half, so the stator's power output is decreased by
half.

I use Tarn-X, a silverware cleaner that can be found in drugstores or
supermarkets. After cleaning the corrosion off the terminals, I flush
the Tarn-X off with distilled water.

Tarn-X will remove the tin plating from electrical terminals if you
leave it on too long.

Somebody else recommended a product called DeOXit that supposedly works
well.

I found a 35-amp single pole single throw toggle switch and mounted it
on a bracket on my handlebar.

The red, blue, and yellow crimp eyelet terminals you can get in auto
parts stores are especially bad. The Japanese used bullet and socket
connectors. When I replace them, I solder them on.

Motorcycles and cars and airplanes use stranded wire instead of solid
copper wire because it's flexible.

But the crimp type terminal only manages to use the outer strands of
wire where they touch the terminal metal, but soldering terminals on
makes the electricity go through ALL the strands of wire.

Ordinary transformer wire from Radio Shack doesn't work. The enamel
insulation won't survive being bathed in hot oil. The Japanese
manufacturers used a special tough green plastic to insulate stator
wires.

 

I went over it with a voltmeter last night, and the bike was getting
power up to where it starts going under the gas tank towards the
handlebars. I took the gas tank off and gave everything under there a
good check over and wiggle test but everything looked good. I gave all
the wires up around the handle bars/behind the headlight a good wiggle
test. I checked the three fuses it has for the main, headlight,
taillight and all those were good. I just talked to my mechanic though
and he said to check the fusible link by the starter solenoid. I'll be
checking that when I go home for lunch.

thanks

 

Do you have 12 volts at each set of ignition points when when they are
open?

If not, check back to the + and minus terminals on the coils. Then, if
there's no power there with each set of points open, go back to the
kill switch and the ignition switch.

I'm not sure which one is first in the series, kill switch or ignition
switch on your machine.

But the ignition switch will be powered by a wire coming from the main
fuse and from the battery, and it will energize a wire that goes back
to the fuse box to power everything that works when the key is turned
on.

I have had iginition switches melt inside and have had other handlebar
switches corrode so I couldn't start the engine.

 

Thanks everybody, with your help I found the problem. The little
switch at the base of the ignition it just old and worn out. Once I
found out the ignition was getting power, I started to try and unplug
the wiring harness from the ignition. The previous owner had it zip
tied in place so after I cut those and started to giggle it out, I
started getting a power flicker. So I took out the ignition, cleaned
the connections and put everything back together, and it seems fine for
now. Just in case however, I ordered a new base to the ignition, and
will be putting it on as soon as possible. So it turns out it willd
cost me less than $20 to fix my problem.


thanks again

 

I had the same problem with my 78' CB750. I dug thru one of my old gun
box's and dug out a 22 bore brush, also got some fine sand paper 220
grit and some electronics cleaner (spray type).
Then I went thru every connection one the bike.
BTW: I found the a digital camera really goes along way when doing
the connection inside the headlight bucket. That can be a real horror
show without some kind of guide.

But lo and behold when I had finished my electrical problems
disappeared.

Good Luck to ya

 

On my CBs, the ign switches have both died over time and exhibited the
same scenario. (Was worse, every time you turned bars to left, it quit)
Replaced ign switches. (25 each from CRC or PnM) All good now. Check the
plugs and connectors. Good luck.

--
Gopher Greg
'77 CB750K Stock '78 CB750F AHRMA
'00 ZG1000 Stock '96 Ducati 900SS Former track bike
'01 GSXR750 Current race bike
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