CBX 750F - revival (now carbs)

[Alemaog4T]

Hi everyone!

first post, already asking for your wisdom

My CBX 750 F has arrived home non-running (e.g. on a trailer). With the battery charged, I turned the ignition key and notice two unsettling things:
i. the dash light "tail light" would light it up whenever the clutch is pulled in (e.g. the clutch switch is "closed").
ii. the run-stop switch does NOT kill the start button (e.g. engine cranks regardless if it is in "run" or "stop" position).

Assuming I have an electrical issue somewhere, I started focusing on diagnosing this. So far, I have tested the continuity of the fuse box, and both right and left controls, and everything checks out with the wiring diagram. During these tests, I found out an even funnier issue: with the key in "on", there is not a single light in the bike that works except the neutral and oil dash lights. Now, with the clutch pulled in (and the "tail light" on the dash on), the park, low and high beams work (including the dimmer), as well as the braking lights

. The blinkers sort of work, with a behavior like there is a burned bulk (although all bulbs light up). I have removed the rear relay light box, and nothing changed except that the rear lights stopped working. I have cleaned all connectors, checked for visual damage on the wire harness - so far, nothing obvious explains this... I have studied the wire diagram for hours now, and cannot see an obvious place where wires could cause such behavior, although I know something is not right...

Note that this is my first "full rebuilt" project, and electrics is the place I have least experience/knowledge.

I am sort of lost on what to test next - has anyone had similar experiences, or would have any advice on what to do next?
Thank you all in advance for your help!

 

[Nylundarn]

Hi! Welcome to the site, I saw your post on the CBX forum!

I do not have the same bike as yours, but the electrical stuff is pretty similar.

Regarding your run/stop switch, Im not really sure but I think it is supposed to be like that. The switch maybe only kills the ignition but not the starter relay. If you attach your wiring scheme Im sure we can figure it out.

Your lighting problems could be a bad ground.

 

[Nylundarn]

Not sure if this is the same as your wiring scheme, but here you can see that the stop/run and starter switches are parallel to each other. The stop/run only take away the ignition circuits power.

 

[Nylundarn]

Okay hear me out,

If your main harness ground is bad, the system needs it from somewhere else. It COULD be that the circuit only gets ground if the clutch lever is pulled (closed) AND the neutral gear is selected, that way all the ground the circuit gets is from the neutral switch.

To test this theory, repeat your test :

1. Key switch on ---> neutral gear selected and clutch lever pulled ----> does the lights work?

2. IF they do ----> keep holding clutch lever closed and switch to 1st gear ----> if the lights go out then my theory could be right.

 

[Nylundarn]

That would also explain why the "tail light warning" light in the dash only lights up when the clutch is pulled, if not pulled "stop and tail light warning sensor" unit gets no ground.

Remember this is just me thinking....

 

[Alemaog4T]

@Nylundarn,
Thank you for taking the time to share your thoughts!

I have just tested your theory above and, as you predicted, the lights stop working when the clutch level is pulled and the bike is switched out of neutral.

Your theory also explain why the lights stop working when the silicon rectifier on the G/R and LG/R wires is removed.

As far as I could find, the main ground is located at the holding bracket from the coils. Is this correct?

 

[Nylundarn]

Great! Yes if I remember correctly, the main ground would be somewhere in the middle of the harness, most often a green 5cm long wire with a ring terminal, make sure to clean and tighten it. Use some sandpaper to remove paint to get to bare metal.

 

[Alemaog4T]

Just passing by to say that re-doing the grounds of the harness eliminated the problem - now, time for fuel-related things. Thank you once again, @Nylundarn!

 

[Nylundarn]

No problem! Happy to help, good luck.

 

[Dustproof]

On the wiring harness there are two Diodes, one is under the tank, the other is inside the dash/fairing. If the one under the tank isn't working
It will not start unless the clutch is pulled in.... it's for the side stand and clutch switch. The other might be your issue, it has to do with the lights.
These Diodes are readily available and not expensive, I keep a couple for my CBX750F just in case. To check if they are working or not, pull them out
and see if anything has changed if it has changed check them with a multimeter. They are a small Black relay looking thing with 2 contacts and plug directly into the harness connector. it is just like testing any diode, power will flow only one direction

 

[Alemaog4T]

Hi everyone!
now, next chapter: the electrical issue mentioned above is fully solved (thanks once again for the help) and I managed to start the bike! It ran like crap (aux fuel tank with new gas, and carbs emptied prior), but it worked - well, sort of: cylinder 2 did not fire.

Now, I have performed the following tests:
1. coils and cables resistance according to manual - all OK
2. spark plug gaps - all within specs and slightly blackened
3. spark on all cylinders - checked
4. compression test: all 4 cylinders within spec WOT.

The previous owner mentioned that the carbs were messed with prior - considering that I have compression and spark, my thought was something is wrong with fuel delivery, namely, the carbs. I removed them over the weekend and the first thing I noticed: the idle screw was not even close to be regulating idle. I guess it is fair to say that someone messed with them before .

I plan to disassemble, inspect and clean them (visually, they do not look like the worst set of carbs). For now, no full rebuild planned. The question: what could cause cylinder 2 not to fire from a cab perspective? I want to make sure I pay close attention to its carb.

As a side note (may or not be relevant): cylinder 2 is the only one that has the pilot screw at a different setting. I am also thinking a reset to the 2 turn value (manual) and fine tuning on the bike would be something to do.