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Melted wiring and no spark

Sully

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Hello,
I am the new owner of a 77 cb750k. The bike was not running when i picked it up. I put a battery in it and changed the oil an tried to start it, and it would turn over but would not fire up. I pulled two of the plugs and checked for a spark but nothing. After further inspection i found that main wiring harness looks melted down to the two white connectors behind the left panel under the seat. I am an ok mechanic but have no electrical experience. Any advice? Should i have a pro rewire it or can i do it myself? Thanks
 
You can get the whole wiring harness from Vintagecb750 for 90 bucks, Just unplug and plug one terminal at a time or keep notes on the order You remove them. You should be able to pull it off, Good luck.
 
Thanks. After further investigation the melted wiring is isolated a bit. I think i can replace the melted wires but Im not sure what the names of a few of the parts are. I know the couplers but not the black box it goes into. The most damaged wire is the green one that goes to my ignition box i think. I have attached a few photos. Thanks again. Box 1.jpgBox 2.jpgBox 3.jpgCoupler 1.jpgCoupler 2.jpg
 
I might not have the best advise but now would be the time to up grade to a new solid state rectifier/regualtor.
 
these old ones are popping up with wiring issues right n left now it seems. a solid state system is good,,the ones from a 79 early 80's can be used in addition to the aftermarket stuff.it is a cleaner setup but their is an advantage to a early system though in being that the voltage can be adjusted at the regulator. truthfully i have never seen a regulator/rectifier that could not be salvaged and work again. the pics you posted,,the black piece is a rectifier and the gold colored is the regulator. it is difficult to diagnose from pics but just guessing,since the green that goes to the rectifier seems to be melted,,,the wiring to that comes directly from the alternator so possibly the alternator wiring grounded out somewhere and caused that..i would start at the alternator wiring and follow it from there.

stock wiring is confusing to me so myself i would just cut all the old wiring off leaving some leads to splice into and redo it all following a simplified diagram. it takes time and patience but the reward is being familiar with every single wire on your bike and being able to diagnose it from there. even if you do not do that here is a simple diagram to look at and get a idea of the relation the wires have to each other. if too small just click and it will open larger.

simple_wiring_diagram.jpg
 
Losttravlyr gave You good advice, The only addition I could add is if Your going to splice wires use good solid solder connections not crimp type butt splices they tend to fail, Also ohm out the alternator and field coil to eliminate damage there.
 
a word about crimp connectors. solder is the best way to go but if you cannot then do this. first use qaulity butt splices from the electrical supply house not the p.o.s. 3m stuff at wally world and auto zone. second,,carefully use an exato knife and peel the plastic insulator back till you can pull the metal part free,then before you crimp it slide a couple small lengths of shrink wrap over the wire then once crimped slide it over the metal and heat it till shrunk around the splice then the other atop that just to be safe. always use the crimpers that look like pliers, they cost a few dollars more but are many times better than the multi hole cheapies.
if you don't solder your stuff that is about the best you can do when crimping and it will last a bit before you have to redo it.
 
I would agree with the comments above. Sounds like something is grounded at the rotor/stater (aka alternator) assembly. If that issue is not addressed, the wires will just burn up again.

If that rectifier is burnt out, you have two options; upgrade to a solid state one (~$100 new) or you can assemble a rectifier from parts from radio shack or any electrical supply store. A 25 amp full wave bridge rectifier can be bought for about $3-$10. You can either use two 2 phase rectifiers (not using one phase from one) or buy a single 3 phase rectifier. It sounds complicated, but not really.
In the image below, grey wires are the same as the three yellow wires, then red is positive and black ground. Just converts AC to DC current.
Recifier_zpsb74ca400.jpg
.

You can get new connectors from http://vintageconnections.com/

As for the rest of the wiring, you can either replace it in parts or buy a new harness. On old bikes, I like just replacing the whole harness. It's way cheaper and much less a pain in the ass than building one yourself (unless your doing a minimal harness for a chopper like above). Soldering, shrink wrap and good quality connectors are your friend. There's a lot of shaking going on on a motorcycle going down the road, you want that wiring to be secure and well insulated.
 
Thanks for all the great advice. I am in the process of replacing the burnt wires and getting new couplers. After I bothered the previous owner a bit I found out that he accidentally hooked the battery up backwards for a few seconds and it started smoking. Besides wiring replacement what else I'm a looking at as far as damaged parts. The three fuses are still in tact. I'm going to replace all just because. Is there another fuse box? Thanks again you guys know your stuff!
 
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