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.