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SOHC.co.uk Forums => CB750 => Topic started by: adriangsmith on March 31, 2016, 09:32:10 PM

Title: CB750 electrics
Post by: adriangsmith on March 31, 2016, 09:32:10 PM

Hi all,
I've just got round to connecting the electrics back up on my 76 cb750f1 following a strip down and rebuild.

Connected up and all working fine until i tried the electric start button.
Headlight was working and neutral and oil light were on.
The solenoid buzzed / clicked and then all power cut out.
Power returned about 20 seconds later but again went when i pressed the starter button again.
Happenened as above twice more and now i have no power at all.
All fuses are OK.
Earth all OK up to handlebars and ignition switch
Still have power to the fuses but nothing further on.
No headlight or idiot lights.

The starter worked fine before the strip down.

Any ideas??

Title: Re: CB750 electrics
Post by: hairygit on March 31, 2016, 09:42:19 PM

Check the earth lead carefully, did you remove paint/powder coat where the cable attaches to the frame?

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Title: Re: CB750 electrics
Post by: adriangsmith on March 31, 2016, 09:47:27 PM

Yes powdercoat ground back down to bare metal.
Good earth throughout

Title: Re: CB750 electrics
Post by: hairygit on March 31, 2016, 09:59:47 PM

Try making a temp earth with a car jump lead from battery earth to a clean bolt on the frame or engine to eliminate a Dodgy earth cable. If that doesn't help, check the plug to the ign switch for power and security, I've known pattern ign switches start to come apart from the lock body and break the circuit.

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Title: Re: CB750 electrics
Post by: AshimotoK0 on March 31, 2016, 10:01:02 PM

Try shorting out the starter solenoid main terminals with a heavy duty cable and see if it turns over then. If it does not suspect earthing or main lead to starter for resistive contact.

Title: Re: CB750 electrics
Post by: Spitfire on April 01, 2016, 09:13:24 AM

I had a similar problem on my F1, it all looked fine until I looked at the back of the fuse box which was corroded to hell, I replaced it and have not looked back since. Hope yours is as easy to fix.

Cheers

Dennis

Title: Re: CB750 electrics
Post by: adriangsmith on April 01, 2016, 09:32:18 AM

Thanks all,
A bad earth sounds the most likely issue.
I also will check the condition of the rectifier.
Also what is the small plug in aluminium block located by the indicator relay? some sort of safety diode??
I'll check when i get home tonight.
Fuse box is in good condition.

Title: Re: CB750 electrics
Post by: adriangsmith on April 01, 2016, 04:55:48 PM

Bad earth was the cause. All sorted now  :) thanks all!

Title: Re: CB750 electrics
Post by: diabolics on April 08, 2016, 08:43:48 AM

As they say. Classic bikes have 3 types of electrical problems.  Bad earths, Bad earths and Bad earths

Title: Re: CB750 electrics
Post by: mike the bike on April 08, 2016, 05:28:55 PM

Another gardening mystery solved; bad earth