Topic: Strange intermittent electrical fault

[fred]

Hello,

My 1978 CB750F2 Supersport (with a 1976 F1 engine and Newtronics electronic ignition installed), has recently developed a strange intermittent electrical fault.

Sometimes the bike won't start with the starter motor. And when I say it won't start , I mean absolutely nothing happens when the start button is depressed. But after kicking her over with the kick starter a few times, the starter motor works and the bike fires up.

Also, the bike will sometimes suddenly cut out, either whilst i'm warming up the engine before zooming off, or whilst i'm riding her for a few minutes. And it's a very sudden complete cut out, as if a fuse has blown. But the fuses are all ok.

It's not a fuel issue, as the bike doesn't cough or stutter. The lights and horn work ok in all instances.

To me it sounds like a dodgy connection somewhere. But where? Any ideas anyone please?!
Thought I had better ask here first instead of aimlessly fiddling around with the wires and connectors. I'm not all that great with bike electrics, but sure i'll be able to sort this out if someone can point me in the right direction.

Thanks in advance for any help,
Cheers,
fred.

[lrutt]

Some EI's are very very sensitive to voltage level. I've seen where using an electric start will draw voltage down enough that some EI's won't trigger. But kicking will be ok.

Same could be for sitting during a warmup.

First thing I'd check is voltage levels, making sure battery is good as is the regulator / rectifier. And make sure you have no voltage drops to the EI.

[TwoTired]

On the back of the key switch is a plug.  It is supposed to have a plastic latch to hold it in position.  But, when the plastic nears its 40th, it looks to retire.

So check that plug .  If it comes loose, a disconnection turns off the bike.

But, really there are a lot of places an intermittent can occur; back of fuse box, corroded loose battery terminals.    A good thorough visual inspection is a good first attempt at repair.  Then connect a voltmeter to a black wire connection and ground , turn on the key switch and start wiggling anything electrical til you can make the power come and go.  Then you know you are getting close.

Cheers.

[evanphi]

Could be a hackeded up ground connection.

I had a PO'd wiring harness with all the ground connections mounted to the battery neg terminal. One came loose and I lost all my gauge functions, but the bike stayed on. I then rebuilt that connection.

There's pics in my build thread of what I had to do.

[strynboen]

as two says..the backside of the main key..thek for meldet plastic konnektor block..often it gives perodic errors..

[HondaMan]

Check the fuses and fuseblock, as follows: remove the fuses (with the battery disconnected) and clean the fuse clips with a .22 caliber gun bore (or similar) wire brush, or fine emery paper wrapped around a small dowel the size of the fuses. If the clips are a chocolate-like color, they have lost their chromate (common at 20+ years) and act like a resistor, lowering the bike's voltage and heating the fuses (sometimes enough to melt them).

The Start button may also be warped, especially if the bike has a halogen headlight. The only fix is a new switch, for this one. I also can make you a Relay Kit (or show you how to make your own) to remove the heavy headlight current from the [new] switch to make it last many years longer.

And, like TT said above: the main contacts in the keyswitch may just be giving it up!

[PeWe]

My K6 has a small wire almost not visible with bullet plug connection going from the battery to starter relay. Corroded connection and the starter did sometimes not work. I think wire is red/yellow.

[fred]

Wow! So many replies in such a short space of time! Thank you so much for the input.
And all your suggestions make sense, especially the use of a voltmeter whilst wiggling certain wires and connections. I'll give all of your ideas a go later today, and let you know how I get on.

Cheers,
fred

[AndyH]

As has been alluded to, many electrical gremlins are due to poor connections.  My 1st port of call would be to check the earths. 

[Keith]

My 750k2 would die on the road with no apparent reason, with a blown fuse each time. Install new fuse....all good again for a while. New fuse block...years later, still good.

[old76cr]

Try fliping  the kill switch about a dozen times and spray some wd40 under it.

[fred]

Well, it's taken a few days to find the fault, but i've finally tracked it down. But why is it always the last thing to try?! lol

First of all I removed the headlight and disconnected everything that lurks within the headlight cowl, cleaned the contacts and applied some ACF-50 before re-assembling.
But alas no spark.

Swore at the bike, had a cup of tea and considered having another go the following day.

The next day I had a rummage behind the the left-hand side cover, and did as mentioned above. Still nothing. Stared at the Newtronic electronic ignition switching unit for a bit and wondered if that could be the culprit.

A bit more swearing, more tea (this time with some biscuits), and thoughts for the next day.

And on the 3rd day, took the fuel tank off and checked connections to and around the coils. Still nothing.

But whilst the side cover was off and I was pressing the starter button, I tapped the Newtronic switching unit with my finger a few times and the bike fired up and then cut out.

Aha!

So I removed the Newtronic switching unit, took it apart and look what I found...

Front of the Newtronic switching unit:


Rear of the switching unit:


After removing the outer rear cover, and revealing the inner plastic cover with some tell-tale brown marks in the bottom left corner:


After removing the inner plastic cover and revealing the circuit board and frazzled components in the bottom left corner:


Close-up of the frazzled components:


I'm going to get in touch with Newtronic and see if they can supply a new board or the just the components that need replacing, unless there's an electonic wizard on here who knows better and can help.

In the meantime, I think i'll put the contacts back in so I can use the bike.

Thanks again for all your thoughts and ideas. It helped to get my confidence up and get working on the bike.

I'll let you know how I get on.

Time for a cup of tea now.
And some cake :-)

Cheers chaps,
fred

[evanphi]

Glad it was figured out.

Your tea-drinking-thinking-process sounds just like Bad Obsession Motorsports and Project Binky.

For the uninitiated: https://www.youtube.com/user/badobsessionmsport

[Bodi]

It looks like maybe overvoltage? The two burned things are a diode (easily replaced as these are standard parts) and a zener diode - you need to get the part number from it to replace it, since zener diodes have a specific voltage specification.
The three terminal device could be a few things, so hopefully you can get a part number from it? Likely a transistor or voltage regulator.
The two large resistors that look burnt - not a worry, you use 5W resistors because they will dissipate power. The discoloration just says they did so, it's not abnormal. But having some components heating up that much suggests a dubious design...
I would toss it out, put the points plate back in, and invest in Hondaman's ignition module. That uses the existing points for timing, works very well, and if it ever fails you can easily switch back to basic points ignition.