Topic: 1978 cb750 k7-no spark on any plugs-quick question!

[Mongoose]

Hi- bought a project non runner 1978 CB750 K7 and I'm going through it bit by bit to recommission.
I'm happy to do the mechanical, but not too confident about the electrics!
I've got no spark at any of the HT leads, and I know it could be a multitude of things-coils, condensers, points, bad earth etc and I'm trying to get through things and do the obvious first.
As there is no spark to anywhere, I thought I would start and make sure the kill switch is not the culprit-hard to believe both coils/points etc have gone bad.
Anyway, I've traced the kill switch/start wires back to the headlight and there is a 4 way connector plug that has 2 blacks into the same port, a red/yellow, a black/white and from memory I think brown wire.
I'm really rubbish at electricery, but have read somewhere, if I leave the plug connected ( so starter works) and join the black and the black/white, it bypasses the kill switch.
Is this correct? I don't want to be blowing fuses etc and causing myself more problems?
Also- I am new to this- is there anything else you would recommend I check? (getting back into bikes-my last one was a 12moth old 250 superdream!LOL)
Thank you

[denward17]

Do you have a known good battery?

Are all the wires connected?  What state is the bike in?

Main fuse ok?

BTW, is this a K7 or K8, not that it matters.

[rotortiller]

First thing I would do is look for power at the Black/white wire under the gas tank powering the coils. The two wires going to the kill switch harness can be shorted together for testing.

[newday777]

Welcome aboard Mongoose
Yes the black and black/white are the Kill switch wires.

[newday777]

Rotated and close up

[Mongoose]

Thanks all- I'll try joining the black/black white and see what happens!
It's a 1978 K7 and im doing it from a power pack, so enough battery power! I think main fuse is ok as everything lights up and all other electrics are on!
Newday 777-thank you very much for the wiring! it makes more sense in color! Ive got a black and white diagram and my old eyes just got confused! Where can I get a color wiring diagram please?

[newday777]

Manuals and Technical Documents section,
Hit 3 lines upper right,
Hit 750K,
Scroll down to find wiring and year.

[newday777]

What is the model year on the vin plate on the steering neck?

1977 model year is a K7
1978 model year is a K8

[BenelliSEI]

Print the wiring diagram for your model. Print the colour version. Invaluable.

[Mongoose]

Print the wiring diagram for your model. Print the colour version. Invaluable.

Sorry-it is a 1978 K8 (manufactured 7-77) I keep referring it to a k7 from habit as David Silver spares only list up to 77 on the Uk site!
No update yet- will have some time on her on Saturday!

[bryanj]

Thats because the UK never had the K8 or the F3

[Bodi]

Electricity is not black magic, 12V battery voltage is pretty safe.
You should have a multimeter. Even the cheapest ones will do what you need.

The coils spark when power is removed. Test for spark by connecting your power pack directly to a coil and then remove power, a plug on that coil should spark the instant power is removed.
With a meter or just a test bulb confirm you have power to the bk/wh wire at the coil (key on, kill at RUN). If you do, and got a park with the above test, then your problem is with the points or their wires to the coils, the points open the ground connection for a spark. A meter or test light will read power on the yellow or blue wires if the points are open, they should be closed for about 3/4 of a crank revolution. The points do corrode and fail to connect, or the adjustment could be way out. If the coils were replaced by a PO they may have been connected wrong ... or they lost some of the insulating washers on the connection point.