Weak spark on 2 and 3

[El__burro]

Good morning,
I am struggling with my cb750 k2 as it starts on 1-4 cilynders only,
so far i have:
- Swapped the coils around then replaced with a new one.
- Bypassed the yellow wire to coil 2-3,
- Replaced condenser
- Checked timing and points gap.
- Replaced and regapped the spark plugs
None of these has sorted the problem.
HINT: i noticed that if i short points 1-4 to ground, engine dies, if i short 2-3 it only causes sparks but engine stays on.
Most of time i get no reading from 2-3 with strobo light.
What else should i check.

Thanks
El burro


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[JamesH]

So when you swapped the coils are you saying you still had a spark on 1-4 but not on 2-3?. If so, you know your coils are good..

[K2-K6]

Swap the supply wires to the coils,  these are fed from the ignition circuit that gives battery voltage to coils.

The wires from coil to points are the earth wires which are switched on and off by the points contacting the engine cases when moving.
If with the bike off and point's wires disconnected,  you should be able to meter the wire and point's for continuity back to earth. Pull points open, no continuity. Leave points closed,  should get consistent continuity to earth.
If not,  you've got a points problem. Either the contact surface or the small insulation washers on the bolts that connect the wire to the points arm. If it touches earth here, then you'll get no switching at all.

[El__burro]

"Either the contact surface or the small insulation washers on the bolts that connect the wire to the points arm. If it touches earth here, then you'll get no switching at all"
Thanks K2-K6

I think this is probably the cause of the problem, the little red rubber washer that isolates the screw that connects the wires to the points assembly.
I'll try to reproduce that, should electrical tape do the job?
El__burro

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[K2-K6]

You may get it to insulate but I doubt if it would last long.

The fibre washers usually have a "top hat" type shape so that as well as insulating the clamping surfaces they go through the hole on the points bracket to stop the small bolt touching to metal peripherally and earthing through that route as well.

Usually the material is very similar to a circuit board base material so if you can find something like that it could be adapted by drilling a hole and then cutting round it.

Just trying to think of something to prove that's what is wrong and get it running.  For decent fix though, you need the correct parts as you'll unlikely get a fail during further use if sorted thoroughly.

[El__burro]

so far, its sorted.
I have wrapped some electrical tape around the screw.
i am aware that it is not the final or lasting solution but... do they sell that part?
Thanks
el__burro

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[K2-K6]

Is it that black tape down bottom of photo on the yellow wire?

If so, you could disconnect the other end of the wire and thread some heat shrink tube down it to make a more permanent fix. You just need a bit long enough to cover the damage,  place it in position and heat with hair dryer or similar to get a good result.

I did think it would be around the attachment point though but that looks in reasonable order so as long as you get effective insulation on the damaged section it should be ok.

At least you've now got to the core of the problem and know how to get it running ok.

I'd bend that green wire slightly further away from the base plate (toward the yellow wire) to make sure it can't also track to earth as well.

[El__burro]

The problem was with the insulating rubber  between the screw and the points plate. I have wrapped it with electrical tape and.. "magically" it worked.
Still im struggling to get a strobo light reading on 2-3 ht lead.
Bike runs fine though.
Thanks for your suggestions
el__burro

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