Topic: 1975 Honda CB750F, electrical-starting-running issues.

[Ratbike222]

I just recently purchased my first bike off of a guy i work with. 1975 Honda CB750F.
The bike had been running fairly well, considering there needs to be some carb adjustments made.

heres my problem though. My main fuse now keeps blowing everytime i go to kickstart the bike. The lights will turn on when I turn the key, but the moment I get the bike fired up, everything dies and i blow another main fuse. I took out the fuse box, cleaned the clips real well, and ran through the wiring on the bike looking for a faulty wire. I found that my ground neutral switch wire had been fried, so I replaced it, and repeated. Still nothing. (a new main wiring harness was installed in 2014).

I also killed a battery during this process, and it seems that the bike wont charge the battery whatsoever. I purchased a rectifier/regulator and a new starter relay, and am waiting for them to be delivered. They needed replacing regardless, just because of age.

Also: Side note. While digging through the wire, I found that there is a green ground wire attached to one of the ignition coils (photo attached)... the wiring diagram in my maintenance manual doesn't have this... Is THIS my problem?



 

[Bankerdanny]

OK, we can start with those are aftermarket coils, so they may need to be wired differently. You need to figure out what that wire is.

I am not sure how the neutral switch wire gets fried since, as you note, it's a ground and doesn't carry any voltage.

electrical issues are a pain in the ass. You have to be very systematic about figuring them out. Do you have a multimeter so that you can test voltage levels, continuity, and resistance?

[scottly]

That green wire is the main harness ground wire, which is normally grounded to the frame by the coil mounting bolt. Doesn't look like your problem.

[TwoTired]

I purchased a rectifier/regulator and a new starter relay, and am waiting for them to be delivered. They needed replacing regardless, just because of age.

Um...based on what electrical theory?  Why aren't you replacing the entire bike becauae of age?

The wire harness has a ground distribution (green wires) separate from the frame.  If you reverse the battery polarity connection, even momentarily, then the harness ground often overheats, and melts its own insulation as well as wire insulation of wires adjacent in the wire bundle.  You could also fry the rectifier.

If your fuse parts near the center, it is likely an overcurrent.  If it parts near the ends, it is likely a fuse clip heating issue.

Cheers,

[Ratbike222]

Thanks for the feedback, gentlemen.

Let's see if I can answer all your questions.

I went through the folder of papers the guy gave me when I bought the bike off of him. The main wiring harness, headlight harness, ignition coils, were bought new and installed in 2014.  After taking a second look, that green wire is indeed the main wire harness ground. So i can rule out that its placement is incorrect. Thank you guys for that info.

Yes I do have a multimeter. The problem is, Im not very experienced with testing voltage levels, continuity, and resistance. Not to sound completely ignorant, but theres a lot that Im unfamiliar with when it comes to setting the multimeter correctly and testing what needs to be tested.

Yes, my fuse is blowing in the center of it. So i can assume its an overcurrent (twotired).

I have not reversed the battery polarity.. Ive been extremely careful when I disconnect and reconnect the battery.

The neutral switch lead, having been fried, has been my main suspect of how Im here right now. Based on zero electrical theory, and strictly following the wiring diagrams/ the wires themselves, while also keeping account that my battery continuously drains, Ive theorized that my problems could lie within the charging system itself. Hence why i just bought new rectifier/regulator. I threw in a starter relay because its oxidized beyond belief, and the main fuse connects directly into it.

I realize that they might not be my problem and that the stator might be the culprit, but I popped open the alternator housing to take a peek, and nothing "stood out" on the stator to where I felt the need to dig deeper. I know this will need multimeter testing and it could actually be my problem.. I just have to figure out the proper way to do all this.

Im literally following every colored wire I can pinpoint, since most are wrapped up on the main harness. Tomorrow, or whenever Im not frustrated enough to punch a hole in the wall, Im going to follow the other wire connection to the main fuse that leads to the ignition switch, and go from there.

Also: What/Where is this "diode" thats connected to the neutral switch. Ive been reading that those can cause issues and are easily replaced.

Appreciate you guys for helping.!!  Seriously

Attached is more pictures, if anything stands out...

[calj737]

One obvious flaw in the bike is the extremely long fuel line dangling below the carbs with an aftermarket inline filter. That will cause you problems with fuel flow.

Someone obviously has altered that frame. The sidestand has been cut off and welded farther back.

The diode you asked about is under the left side panel, dangling to the right of the Regulator (that’s the rounded oval thing vertically oriented).

A Main fuse that blows once the engine runs can often be traced to a chaffed wire between the switch controls at the handlebars. You need to diligently inspect those, open the headlight bucket and inspect those, and beneath the instrument cluster.

I doubt there’s anything wrong with your stator. Once you get your fuse issue resolved, it will be much easier to diagnose the actual cause of the charging issues.

[TwoTired]

Stators rarely fail.  I've never found one bad that hadn't met with a road incident, or a crazed attacker with a firearm.
But, to check, set you multimeter to the lowest ohms range, and press the probe tips together.  Note the reading, this is your meter error and this value must be subtracted from all your ohm measurements. 
The spec is something like 0.35 ohms between each of the three windings.  Usually need specialty meters to measure this with accuracy.  But, if you get something low for a reading and they are all the same, then you only need to verify that measuring near infinity between windings and frame work, before checking off the stator as a possible issue.

The field coil should measure 6.8 ohms for a 750.    The shop manual has these values listed.  10% away from spec will still work.

Only blows when running is likely a chafed wire.  Physical inspection, or isolation of unnecessary circuits in turn eventually locates the branch and then specific cause.  You could try replacing the fuse with and incandescent bulb (I use a handy 12v instrument bulb with alligator clips attached), which will limit the current flow to safe levels.  Activate the key switch, and then watch the bulb brightness while wiggling the wires. When the bulb gets brighter, you know you are in the right area of the short and can scrutinize further to pinpoint cause and correct.

No, you can't run the bike with the bulb in place of the fuse.  But, the bike doesn't normally draw more current when running than not.  Normally the bike draws about 10 amps through that 15A fuse with lighting switched on.  Of course, if someone replaced the stock 50W headlight with a 300Watt, that will fry the fuse when the start button reconnects it after e-start.

As to your harness, make the wire connections match the wire diagram.   It's pretty reliable once that's sorted.

Cheers,

[Ratbike222]

Thank guys for the imput.

Im looking at this wire diagram, and I noticed that i should have only one connection to my positive battery terminal.  I have two. 
The diagram shows one red going from the battery to the starter relay.  This second one is a mystery as for now.

This second wire (not to the relay) leads into my main wire harness and gets lost in the shrink.  Either way, I disconnected it by its m/f connection (circled in red/picture)  and found part of my problem. ( 2nd pic)

I feel like i should probably chase those wires to find out where they are leading to, and def replace.  Seems to me to be a 14 gauge wire. Can i just splice and replace this part with similar sized/coated wire?

[Ratbike222]

I removed the casing and shrink from this wire.

[Ratbike222]

Wire Diagram

If I follow this diagram correctly, and tell me if im mistaken here, that this second mysterious red & red/white wire connection should also be connected to the starter relay. 

If this is the case, then my connections between the fuse box & rectifier have been mistakenly attached to the positive battery terminal. Which would then clarify why I blowing the main fuse. ?

[Bankerdanny]

The power lead from the battery runs to one side of the solenoid and a wire comes off the same solenoid terminal to the fuse block.

There is a yellow/red wire from the starter switch that runs to the solenoid area where it has a female bullet terminal that connects to the same color wire that then feeds into the solenoid directly. When you press the starter button the yellow/red wire feeds power to the solenoid, activating it and sending power from the battery through the solenoid to the starter. Part of how I test for a starter switch issue is by disconnecting the wire and touching the male bullet on the solenoid side direct to the battery. If the starter spins the solenoid and starter are fine and you have a starter button issue.

[calj737]

Wire Diagram

If I follow this diagram correctly, and tell me if im mistaken here, that this second mysterious red & red/white wire connection should also be connected to the starter relay. 

If this is the case, then my connections between the fuse box & rectifier have been mistakenly attached to the positive battery terminal. Which would then clarify why I blowing the main fuse. ?

Maybe. The Red and RD/WHT certainly should not be connected directly to the battery. They are the power source for the relay, allowing the battery + terminal lead to transfer that power to the starter motor. The solenoid (relay) needs the Red/WHT and the Y/R (from the START button) to trip the contacts within the solenoid to allow the power to transfer.

It’s plausible that if the solenoid wires were directly connected to the battery + you’d blow a fuse... but honestly I’d want to put a meter on those wires in different conditions before I claimed that to be true. Too late here, perhaps a bit too many adult beverages to think it through. But you should do what you can to restore the proper wire colors to the proper locations to help diagnose what is wrong. It’s a bear when POs switch stuff around and you’re undoing that to get a baseline to work from.

Patience and diligence will see you through. Keep working from that colored diagram and you’ll make lots of progress and save yourself tons of money.

[Ratbike222]

Update. Got my new parts in yesterday, went through all the wiring, and now its set up. (pic)

I connected the battery and tested it with a multimeter. it reads 12.8 turned off, and when i turn the key (without starting the engine) she holds at 12.33.

Im going to attach the gas tank here, start her up, and see what happens....

When I start her up, what should the multimeter read?

[seanbarney41]

Someone obviously has altered that frame. The sidestand has been cut off and welded farther back.

not the case...frame looks completely stock for 1975 cb750f.  Sidestand location is as Honda supplied it.

[SohcCBs]

All good advice, but it sounds like you're heading in the right direction.  Electrical issues take time and YOU have to figure it out yourself.  Don't get in a hurry.  If you don't have a honda service manual, get one.  I think they have some available for download on this site.

[Ratbike222]

Update. She turns on and stays running without blowing a fuse!

The battery is holding charge also.

now I just have to do some carburetor syncing and she should be good to go! 

[calj737]

Someone obviously has altered that frame. The sidestand has been cut off and welded farther back.

not the case...frame looks completely stock for 1975 cb750f.  Sidestand location is as Honda supplied it.

If you actually look at the picture, you'll clearly see the front area of the frame tube has had a modification removed. The side stand is in the stock location, but it has been TIG welded, not MIG welded a la stock.

[seanbarney41]

Someone obviously has altered that frame. The sidestand has been cut off and welded farther back.

not the case...frame looks completely stock for 1975 cb750f.  Sidestand location is as Honda supplied it.

If you actually look at the picture, you'll clearly see the front area of the frame tube has had a modification removed. The side stand is in the stock location, but it has been TIG welded, not MIG welded a la stock.

oh yeah, that weld is wayyy too pretty for Honda factory...not sure what we are seeing at the front, is that not the front lower motor mount?

[Bankerdanny]

I can't comment on weld quality since my welds uniformly sick, but the stand position appears correct. Here is my F1 from the left side