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Author Topic: 1973 CB750 K3 Build Thread  (Read 2060 times)

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Offline StealthCB

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1973 CB750 K3 Build Thread
« on: August 19, 2018, 05:35:07 AM »
Hey guys, I bought a beat up, not together 1973 CB750 a few months ago, in hopes of bringing it back to life. With help from this forum and tech write ups, I properly adjusted my timing and tappet clearances. I installed new NGK plugs and caps, reproduction petcock, tore down and cleaned like crazy my carbs and installed new bowl gaskets, installed black superbike bars, bought new cables, simple throttle assembly, and brown grips. I also am planning to use a set of 1980 CB750k forks and brake caliper and master cylinder, as the stanchions on the K3 forks are rusted beyond repair.

I have a rare 4 into 1 Bassani dohc exhaust that I want to try and get to work. It currently has a 4 into 2 that was chopped, so they are open pipes. I got it running a few days ago and it sounds so mean with open pipes, but I don't plan to ride it like that. The picture is from when I picked it up. The guy gave me a seat that is for a totally different Honda. I also just bought a 2005 CBR1000 start/kill switch that I am working on figuring out how to properly wire up.
« Last Edit: August 19, 2018, 05:38:52 AM by StealthCB »
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Offline krbuck

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Re: 1973 CB750 K3 Build Thread
« Reply #1 on: August 19, 2018, 07:46:43 AM »
Wow, I feel you are pretty brave to take on a bike looking like that. Very inspiring to me - I'm looking for my first CB750 project.

Do you have an end goal? Do you want to get it up to daily runner condition?
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Offline StealthCB

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Re: 1973 CB750 K3 Build Thread
« Reply #2 on: August 19, 2018, 10:40:56 AM »
Haha! Thanks! Well, I bought it for next to nothing, as I have next to no play money, but I love working on stuff and bringing things back to life. I've run into some headaches, like way over tightened drain bolts that stripped, and the oil filter mounting place on the block being JB welded for some reason, but I'm enjoying the journey.

Yes, my plan is to make it safe and enjoy it a few times a week or so.

Here is a quick video, from my Instagram build account, of it running.

https://www.instagram.com/p/BmZUZnNAbCf/?utm_source=ig_share_sheet&igshid=1ud4toiz24ujc
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Offline StealthCB

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Re: 1973 CB750 K3 Build Thread
« Reply #3 on: August 19, 2018, 05:59:19 PM »
Alright, so I went up to the shop today for a good while, took apart the old Bassani exhaust, which, sadly, looks to be a total goner. The next thing I worked on was everyone's favorite... Wiring!

So, I definitely need some help from the experts. I have been studying the wiring schematic and checking various things for a while now. Today I worked on wiring up the CBR kill switch. I checked continuity first, and everything checked out for the start button and kill switch. Hooked it up, hooked up the battery to the wiring harness, then... nothing. Pulled out a test light and the only thing showing power was the red wire on the back of the ignition switch for "IG" and the brown wire for "TL2".

So, to problem solve and see what should get power and all. I hooked up 12v directly to the black wire that should plug into the starter selenoid. Two black and two green wires, all with two female ends, have power, which is what I need to accurately check the CBR switch. I even swapped out another starter selenoid but nothing.

My question then, what do I need to check or replace? What am I doing wrong here that I am not getting any power to wires when the battery is plugged up to the harness?
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Offline calj737

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Re: 1973 CB750 K3 Build Thread
« Reply #4 on: August 20, 2018, 02:22:07 AM »
Stock ignition switch? If so, see if you have continuity between red and black with key ON, no battery. If not, your switch is bad.
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'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline StealthCB

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Re: 1973 CB750 K3 Build Thread
« Reply #5 on: August 20, 2018, 04:26:19 AM »
Yes, it is. I will check that. Thank you so much!
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Offline calj737

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Re: 1973 CB750 K3 Build Thread
« Reply #6 on: August 20, 2018, 04:59:50 AM »
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'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline StealthCB

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Re: 1973 CB750 K3 Build Thread
« Reply #7 on: August 20, 2018, 06:49:50 AM »
Thank you. That is the exact wiring schematic I have been Consulting. I'm definitely learning a lot as I go, as wiring is not my forte.

I forgot to mention something that might automatically tell you if my ignition switch is bad. When I applied power directly to the black wire, it didn't matter if the ignition switch was completely off or on. I still had power.
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Offline calj737

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Re: 1973 CB750 K3 Build Thread
« Reply #8 on: August 20, 2018, 07:48:42 AM »
It does not. You hav essentially bypassed the key switch by applying popper directly to BLACK. BLACK only should have power when the key is switched from OFF to ON (transferred from RED to BLACK).

You basically showed up to the Dr having cut off your leg below the knee and are asking whether your ankle was broken. No way to know...  ;)
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'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline StealthCB

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Re: 1973 CB750 K3 Build Thread
« Reply #9 on: August 20, 2018, 08:47:16 AM »
Oh, okay. Thank you for the information. Love the analogy, by the way!
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Offline StealthCB

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Re: 1973 CB750 K3 Build Thread
« Reply #10 on: August 25, 2018, 07:04:37 PM »
Well, I made some good progress recently. I installed a new throttle assembly and cables, tachometer cable, gauges, and a new to me seat! I also checked my ignition switch, and there is continuity between the red and black wires when the switch is in the second position, but when it goes to the on position, there is not. Guess I need a new switch.

So, I could really use some advice on what to do about my exhaust. They are 4 individual pipes that have two two into 1 mufflers that have been hacked off to leave open headers. I know for a healthy running engine, and to save my ears, I need mufflers of some sort for back pressure.

With that said, if you were me and on a tight budget, what would you do? Make aftermarket, roughly 2.25" ID, mufflers work, search for used slip on mufflers, or buy some pipe and some baffles of sorts to customize? Thanks guys!
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Offline StealthCB

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Re: 1973 CB750 K3 Build Thread
« Reply #11 on: September 15, 2018, 04:06:11 PM »
Hey guys, it's been a while. So, I bought a replacement ignition switch and got up to the shop today. Installed a battery and routed the wiring harness, then I plugged in my used CBR1000 stop/start switch, which is in good working order, and tried to start it. Nothing! I pulled out my multimeter and checked a ton of connections. Here's what I got.

In the first position I got 12plus volts to the black lead with the stop/start switch on. With the kill switch engaged it kills the power to the coils, great! Turn the ignition switch to the on position and power shows about .4 volts for the coils and black lead. The start button pressed does absolutely nothing, nor is there a sound at the starter selenoid. I'm pretty stumped. Can anyone help me out here?

K3- red/yellow=start black=ignition black/white=coils red=battery

CBR - red/yellow=start black=ignition white/black=battery black/red=battery3

So, everything pairs up and the black/red wire from the start/stop switch is just plugged into the black lead from the harness.

What in the world could be going wrong here!? Any help and advice would be greatly appreciated. Thanks.
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Offline seanbarney41

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Re: 1973 CB750 K3 Build Thread
« Reply #12 on: September 15, 2018, 06:10:42 PM »
not sure you realize this...the key switch has three positions, first one is off, that's obvious, the SECOND position is ON, what you use to run and operate the bike, third is park, that just turns on the tail light only.
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If it works good, it looks good...

Offline 754

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Re: 1973 CB750 K3 Build Thread
« Reply #13 on: September 15, 2018, 06:59:18 PM »
I think you better look into the JB weld on the cases issues .. see what it need there.
 Good luck with the project..
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My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline StealthCB

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Re: 1973 CB750 K3 Build Thread
« Reply #14 on: September 15, 2018, 07:06:16 PM »
Sorry, by first position I meant the position after off. I did not know about the third position, though, for some reason. Ok, still it would not turn over in the second "on" position.
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Offline StealthCB

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Re: 1973 CB750 K3 Build Thread
« Reply #15 on: September 15, 2018, 07:12:38 PM »
Quote from: StealthCB on September 15, 2018, 07:06:16 PM
Sorry, by first position I meant the position after off. I did not know about the third position, though, for some reason. Ok, still it would not turn over in the second "on" position.

I'll get to that, eventually. It runs well, as is for now. I can drain the oil and get that section welded and it'll be just fine, I believe.
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Offline StealthCB

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Re: 1973 CB750 K3 Build Thread
« Reply #16 on: September 16, 2018, 06:53:12 AM »
I caught something for the first time on the wiring diagram. It looks like the original K3 start/stop switch used the yellow/red and a green/red wire for the start button. The green/red wire goes to the safety unit and then the clutch switch, I believe.

So, what wire from the CBR switch would I plug into the green/red wire? Do I actually need this, or should I bypass it? Sorry for all the questions but I really am stumped, and I know you guys know a lot more than me. Thanks!
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