Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Yuppie_88 (Luke) on August 11, 2024, 09:58:30 AM
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Hello everyone, I'm new here :)
I've recently purchased my first CB750 K4, a US import from a dealer. I've done some basic restos in the past, mainly on smaller engines (DT50, CB125, YZ125) but I would consider myself a newbie wanting to learn really.
Anyway, the bike was running at the dealership after being hot-wired and setup on a dummy fuel line. Having got it home I started to investigate the wiring a little further and found that the fuse box and the safety unit were missing, the live battery cable was also corroded (see pics). Replacement parts are on their way.
My only concern is, it looks as though there has been some heat in the wiring at some point - the harness tape looks like it's melted in some places. I can't see any bare copper wires showing, the wire insulation looks ok. That being said I'm still a little concerned about hooking up a battery to it - fire extinguisher at the ready!
A couple of hypnosis I've pondered (this is where I would appreciate your expert input)...
1. Maybe someone at some point, has hooked the battery up the wrong way round, hence the melted tape and the looks of the starter ground wire.
2. There has been some external heat in this area (unrelated to the wiring), hence there isn't any bare copper wire showing (seems unlikely to me though).
3. There is a dodgy ground somewhere in the harness. There is evidence that someone might have been playing about with the indicator electrics and headlight.
Any thoughts or observations are appreciated
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Pics
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I might be out on a limb here but in the past I used to change the polarity on cars pre- alternator days to enable the fitting of positive earth radios. No wiring was melted we used to just reverse the coil wires. Engine still ran without reversing the coil wires. I suspect alternators are different.
More likely poor earth somewhere. If you haven't done so might be worth unwinding some loom insulation to check if there are any of the loom internal joints that are damaged.
When I did a first start up on my 400 after a 13 year standstill I disconnected all the indicators, horn front lights, bar switches so fired her up with just the ignition circuit, alternator, regulator and rectifier in place. Might be worth considering something similar.
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That melting is probably caused by high resistance on the earth lead at engine end meaning starter current flows via the small earth lead bolted under the seat, used to see it a lot in the 70's and now if paint/powder coat not removed where earth lead bolts and engine case touches frame
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I think I have exactly this problem on my CB550 project Yuppie_88
The earth wire that runs from the connector block by the fuses to the tab washer earthed by the coils has melted insulation along its entire length and the spade in the connector block has melted the plastic around it.
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Thats what happens when that small wire carries starter current.
On ebay you can now get Lucas wiring loom smoke for if you have let yours escape!
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Thanks everyone for the replies, it sounds like the first point of call is to check good ground connections to the frame then.
I noted from the wiring diagram; the regulator, rectifier and safety unit all ground via the ground wire bolted to the frame where the seat lock latch is, whereas the starter and starter switch obviously get ground from the engine.
A few parts arrived today so that should keep me busy this weekend. The connector blocks and bullet connectors look a bit crusty in places, does anyone recommend anywhere best to get replacements to keep everything nice and standard?
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Kojaycat do the block connectors.
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Kojaycat do the block connectors.
Brilliant, I'm assuming it's the 3.9mm Japanese bullets?
https://kojaycat.co.uk/epages/950000457.sf/en_GB/?ObjectPath=/Shops/950000457/Categories/Connectors_Bullet/39mm_Japanese_Bullet_Connectors (https://kojaycat.co.uk/epages/950000457.sf/en_GB/?ObjectPath=/Shops/950000457/Categories/Connectors_Bullet/39mm_Japanese_Bullet_Connectors)
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I ordered replacement connector block kits from Kojaycat today. A great recommendation; thanks Bryan!
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Wasnt me, i send people to Big Al
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Oops; it was Ted! I get so many good pieces of advice I am losing track of where they come from. I am hoping to repair the loom/electrics myself but if I decide to go for a new loom then Big Al it is. Thanks again.
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Kojaycat sell the correct latched connectors. Check that the 5 wire (starter motor safety unit) cut-out has been correctly by-passed...red connector.
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Thanks for the tip BigAl. I have to confess I don’t yet have the knowledge of the wiring to understand what I need to do re starter motor cutout. Could you explain please? Thanks
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Try and get hold of 'Hondamans' (Mark Paris) book CB750 he explains the conversion with diagrams very well...pages V-7,V-8
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Thanks. So far the best price I can find is £70.00 plus postage. I will keep looking.
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I have a copy but is hidden in the loft under two weddings worth of stuff.
Happy to dig it out and copy a few pages. Unless anyone else on here has easier-to-access copy?
Mark has these books published in (very) small batches. Hence the price.
Have you asked him?
Steve
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Thanks Steve - I just realised I have confused things. I have a '76 550 K2, not a 750. Although maybe I still need to do something similar?
Apologies for the confusion Alan.
M
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No problem, the CB750 (K3,K4,K5) & CB550 (K0,K1) share the same 5-wire safety switch...the CB550 (K2,K3) have the 2-wire safety switch
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Mark has been in the process of doing a 500/550 book for a few years now, hopefully available in the near future
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No problem, the CB750 (K3,K4,K5) & CB550 (K0,K1) share the same 5-wire safety switch...the CB550 (K2,K3) have the 2-wire safety switch
Thanks Alan, is the safety switch the red rotary switch on the right handlebar?
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Kojaycat do the block connectors.
Brilliant, I'm assuming it's the 3.9mm Japanese bullets?
https://kojaycat.co.uk/epages/950000457.sf/en_GB/?ObjectPath=/Shops/950000457/Categories/Connectors_Bullet/39mm_Japanese_Bullet_Connectors (https://kojaycat.co.uk/epages/950000457.sf/en_GB/?ObjectPath=/Shops/950000457/Categories/Connectors_Bullet/39mm_Japanese_Bullet_Connectors)
My 400 has 3.5 mm Japanese Bullets I think my 500 is the same - I have no idea about the 750 ones.
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No the red switch cuts power to the coils, the safety circuit stops the starter working
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Just for clarity,......stops the starter working....when side stand is down or bike is in gear ( I assume it's the same on this bike as on mine).
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I don't think the the CB750 (K3,K4,K5) & CB550 (K0.K1) had a side stand switch? They do both have the 6-way red connector fitted within the harness for the 5 -wire safety switch...just for reference the plug-in safety switch part No. is 31800-341-701...possibly not available, but it can be by-passed.
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No side stand switch on mine and no red connector block either - all connector blocks are white. I guess the PO replaced the red connector block with a stock white one for some reason. There is a 5 way connector with wires going to the heat sink thing that is mounted above the starter solenoid.
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Yep, on mine (750 K4) there is no side stand switch either.
The safety unit is unfortunately discontinued - I spoke to David Silver today to confirm. This means I'll be jumping the safety unit connection as BigAl mentioned.
I've found a nice electrical box online which has 60mm centre mounts, this should bolt on directly to the original safety unit mount (could be a nice place to keep some spare fuses too). I intend to run the jumper connection through the box to keep things tidy and add a nice new connector block on the end of it. I know I'm the only one who will probably see this but, I don't like the idea of this thing flapping around ;D
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This thread ought to be on the 500/550 section.
My 500 K1 has no red block connectors, no stand or clutch switch.
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Good point Ted. See new post on 500/550:
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I thought the site moderator could just move the whole caboodle.
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I could. Quite easily.
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Clever stuff; thanks Steve!
I have a microswitch in the clutch lever housing, nothing in the sidestand and no red connector. What does the red connector connect to Alan?
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Maybe the red 6-way connector has been removed on your bike?
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Quite possibly. When it is there, what does it connect to?
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It connects to a Safety Circuit Unit but they were changed for a simple diode on later models
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Honda safety switch part No. 31800-341-701
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Al got there first with number, should be screenshot of cmsnl page, and its a 750, 341, part