Topic: what could have caused this?

[78whiteorbs]

Maybe it was the tinfoil around the main fuse....headsmack!

Just picked up a really pretty 1976 cb750 F and the guy must have  knew he had smoked the harness .
We traded and I am still very happy with the trade but look at that.
The original harness black tape was present but there were some splices here and there. Nothing looked wrong though. Called the fella up and he said ,oh yeah I have a brand new harness ,ignition switch and fuse terminal I was gonna sell but you can have em.....ha , what a knuckle head.

That could been real bad if the thing caught fire when I was riding it with a full tank.
Bike has so much brand new stuff. NOS Carbs....30 sohc750's and I have never seen a new pair !
Brand new old stock brakes, gauges read 6,700 miles... it's a very nice looking survivor. Really nice. Just gonna ride this one after I get that harness installed

Word to the wie always take a good look over a new bike for these kinds of things. The life you save may be your own.

[78whiteorbs]

And because everybody loves pics.

[scottly]

This is caused by hooking the battery with the plus and minus terminals reversed. It usually also destroys the rectifier.

[78whiteorbs]

For the win👍 Man that makes perfect sense. First time seeing that one.  I think you nailed it I just learned something.thanks

[Deltarider]

Must have been some generic battery. Fit an original one and it can never happen.

[bryanj]

Also happens if the grond lead to engine is not tight as starter current then goes through that small green ground

[PeWe]

Good information here.
Possible to smoke the electrics without connecting things wrong. I was careful filing paint away from both frame and engine case and applied grease to ensure good ground.

I had mostly a good working starter motor in my mind then. I made a mistake long time ago connecting the ground cable from battery to frame by one of the bolts holding the battery cage.

Yes, I had some starter issues when engine was warm and has been parked 10 minutes. I blamed the 836 with forged pistons. Started bike with kick starter and full throttle. I scared one chick outside an Italian gas station when the loud 4-1 suddenly screamed. She pressed her ice cream into her face looking even better

I did not blow any electric stuff.
I knew better when I restored bike many years later and did it right.

[ekpent]

 No engine number on the vin pad ?  Whats the model number on those carbs. The later Super sport carbs had a knurled air screw with ridges on it that a plastic limiter cap could be pressed on. Seems if they were nos they would have the rubber dust protectors on top also.

[bryanj]

Looks like there are faint impressions there, has it been polished excessively?

[78whiteorbs]

Faint impressions were? Yes he did polish alotta stuff. The carbs ....if they were vapor blasted or something maybe but I have never seen a set this nice. They look brand spanking new. Zero corrosion anywheere.

I'll look for a number.

[78whiteorbs]

Well , took one across the chin today...

Installed new harness, new ignition switch battery was correct. Cleaned contacts and greased.
Hooked up battery and before I could turn the key saw this ......promptly pulled each fuse. WTH???
Smoked the same wire...brand new harness. Matched all the wires with a schematic.
Man that's upsetting...

[78whiteorbs]

More

[Keith]

You have a dead short to ground. Appears that it’s not fused  o/w it would have blown a fuse. Trace this wire.... where does it go to? determine if it should have power or ground, then see what it actually is. Next do this..disconnect your neg. battery cable, place a test light between the post and the cable. If it lights, you have s short... disconnect things until the light goes out. If not lit up, pull all fuses, turn key on. Now try to make the light go out.

[78whiteorbs]

So I did a continuity test with the fuses out and the lighting circuit screams on the (should be dead left side).
Left it screaming and removed a wire at a time in the bucket to no avail. Then removed the green that goes I to the coil as picture and SILENCE. Bingo.
Did a continuity test between the engine case and each coil wire and they all are blaring so it would seem the coils are grounding out somehow???

[Jerry Rxman Griffin aka MuthaF'er]

Please explain your use of the terms screams and blaring

[78whiteorbs]

Continuity tone.

[78whiteorbs]

You have a dead short to ground. Appears that it’s not fused  o/w it would have blown a fuse. Trace this wire.... where does it go to? determine if it should have power or ground, then see what it actually is. Next do this..disconnect your neg. battery cable, place a test light between the post and the cable. If it lights, you have s short... disconnect things until the light goes out. If not lit up, pull all fuses, turn key on. Now try to make the light go out.

Not sure if I understood you correctly. Disconnected the negative as suggested and put a test light in between.
No light....

I dont understand the next sentence....if not lit up pull all fuses turn key on now try to make the light go out....what light?

[bryanj]

If the points are closed there will be continuity to engine case. There is no green wire that connects to coils on a 750

[Keith]

What should light up is the test light. Did you find wHere that wire goes?

[78whiteorbs]

What should light up is the test light. Did you find wHere that wire goes?

The wire that melted is green and was the ground wire that went to the various bucket grounds as well as the coil mount bracket.

I have tested all wires one by one in the bucket  to see which ones short to ground and I found one....it goes to the right control switch. Its black witha. Red tracer. I'd toned for continuity unity to ground and it also shows voltage with the key off 10.3v. So a wire that show continuity to ground and voltage cant be right eh?

I disconnected and its gotta be like a headlight interrupt or something from the wiring diagram I cant figure out what it is exactly. But after disconnecting g it I can turn the bike on and use the starter etc and no smoking ground wire. One other weird thing is I feld the main fuse and one side felt hot so I put a temp gun on it and one side was up to 130 degrees ...is that normal??? All the rest were like 95....doesnt seem right. Also the blue headlight wire in the bucket tones for continuity to ground. Thinking those two wires are related and the answer is there I just cant uncover it. If it's in the bars for the right pod in theory I could replace bars with another set with there pods intact and itd solve the problem. Blah blah blah

[78whiteorbs]

Heres the other wire that was toning for continuity to ground.

Correct me if I'm wrong but the ONLY wire that should continuity tone to ground is the actual ground wire(green) is that correct?

[scottly]

Well , took one across the chin today...

Installed new harness, new ignition switch battery was correct. Cleaned contacts and greased.
Hooked up battery and before I could turn the key saw this ......promptly pulled each fuse. WTH???
Smoked the same wire...brand new harness. Matched all the wires with a schematic.
Man that's upsetting...

Did you replace the original shorted rectifier? The rectifier is not fused, BTW.

[78whiteorbs]

And the "warm" fuse....is that normal?

[78whiteorbs]

Well , took one across the chin today...

Installed new harness, new ignition switch battery was correct. Cleaned contacts and greased.
Hooked up battery and before I could turn the key saw this ......promptly pulled each fuse. WTH???
Smoked the same wire...brand new harness. Matched all the wires with a schematic.
Man that's upsetting...

Did you replace the original shorted rectifier? The rectifier is not fused, BTW.

I did not but I have a spare. My temp probe also showed the green particle coil getting warm on that too. Not glowing red melt stuff hot but over 110

[scottly]

The rectifier got fried before, which fried the green wire on your new harness. Using a temp probe is not really the proper way to locate a short, since by that time even more damage is done.

[78whiteorbs]

The rectifier got fried before, which fried the green wire on your new harness. Using a temp probe is not really the proper way to locate a short, since by that time even more damage is done.

No I wasnt using a temp prob for that. I was using a multimeter set to test continuity by tone. I used a temp probe because the fuse felt especially warm.

[bryanj]

Those fuse boxes, when they get old, are notorious for the clips getting corroded and a loose fit, so much so they can melt the plastic housing and damage the wire connection on the back making it even worse.
Polish the inside of the contact clips and make sure they fit tight on the fuse end, in bad cases replace the fuse box. Most people who do that replace with a blade fuse type.
The black red wire in rh switch should connect to black with starter button released and disconnect when button pushed.
Blue wire goes to headlamp main and blue idiot light(main beam indicator) and should have a resistance not a short. Toning is not a good test you need to measure ohms.

[78whiteorbs]

Yeah knew about the blade fuse block up grade...Does Hondaman sell those?


What does that black red wire do? What's it for? I got no power to headlight ,green(ground), white(lo) or blue(hi).

I was tracing a short that melted my groundwire in seconds. I didnt replace the rectifier  when I installed the new harness as I assumed that circuit was fused but later found out it was not.

Gonna replace that melted wire in the new harness and replace the rectifier and see if that does it.
When I checked the hi beam indicator light was blown too of course.

[bryanj]

Tell me where the other end goes in the switch and i will give you a guess

[PeWe]

Circuit diagram is a must when installing the harness and verify each wire when connecting it.
There are diagrams for download.
http://manuals.sohc4.net/cb750f/

[78whiteorbs]

Peewee I did use a diagram- everything went were the old one was removed. Only 3 extras by the rectifier on the new harnessdont know what they are for. I've I stalled new harnesses before and was pretty straight forward everytime.

[ekpent]

 There are 3 wires that go to the starter button on the 75F. Look on the wiring diagram. When you press the start button it shuts off the headlight to save juice I guess. Black is a power wire.

[78whiteorbs]

Right that's what I was thinking it was. The headlight wires are not getting voltage. I do know that. The bike starts and diesnt melt wires fine as long ad the black with red tracer wire from the starter switch is disconnect from the  starter switch. .maybe that's why no juice to headlight...ah, man I hate electrical probs!

What's the methodical approach here...hellp me out guys. Replace the prolly fried rectifier, that's a given....next...
.
I honestly thought to buy a new set of bars with pods to try and eliminate that from the equation which funny in a way.

[seanbarney41]

try cleaning the starter button with some spray contact cleaner.  If you have to try and take it apart, the plastic is likely to crumble so be careful.  Good thing is pretty much every U.S. market honda from 76-80 or so uses the same right hand control.  Only the wire length varies.

[HondaMan]

Yeah knew about the blade fuse block up grade...Does Hondaman sell those?


What does that black red wire do? What's it for? I got no power to headlight ,green(ground), white(lo) or blue(hi).

I was tracing a short that melted my groundwire in seconds. I didnt replace the rectifier  when I installed the new harness as I assumed that circuit was fused but later found out it was not.

Gonna replace that melted wire in the new harness and replace the rectifier and see if that does it.
When I checked the hi beam indicator light was blown too of course.

Yep, still make them.

The Black/Red wire comes from the Headlight fuse in the fuseblock. This headlight fuse is supposed to get its power from a Black/Brown (some are Black/Yellow) that comes from the un-pressed contact of the START button. This power disappears when the START button is pressed, diverting the power instead to the Start solenoid while killing the headlight.

[bryanj]

I asked to confirm where it goes because there are aftermarket switches out there with the wrong colours  going into switch then the diagram is useless!!!

[PeWe]

I should disconnect all connectors and check everything, functions and look for short cuts. Easy that a connection in handle bar switch shorts to bar or switch housing

Very important to verify each lead on an aftermarket switch where colors are not always exact. Not even switches in a Honda labeled box.
Verify function with a buzzer and mark each wire with tape and note.
Compare with diagram which is a must to follow when working with the electrics.

One thing that usually is missing is the jumper cable from black to brown/white to get the gauges to shine. I do not know if all CB's have that.

Verify everything before connecting battery. At my last build I connected battery plus via an auto multimeter reading the current, a few amps OK feeding coils and light added more when switched on.
Feel if any wire becomes hot was not good enough.
All cabling was new on that bike, aftermarket switches and main harness.

[78whiteorbs]

Thanks for the replies fellas. Ba k at it tomorrow....the control switch appear stock to me. My hunch is that right pod is shorting In the housing or bars but of course have no way to quantify this so I'll be digging back in tomorrow. Cant wait to mark this solved!

[PeWe]

With a cheap automotive multimeter you can measure the current to see where the problem is.
DC 20A

Dwell 8cyl 23-24.4 degrees for CB points gap setting while idle.
Diode tester for rectifier. The diode work in one direction which is most important to verify. Usually 3 diodes in same direction, 4th opposite.  CB750 shop manual ch.8 describe details of rectifier.

Something like this
https://www.ebay.de/itm/352778367986