Author Topic: 1973 CB750 Rescue (Read 5110 times)

j343my · « **on:** December 03, 2022, 03:09:43 PM »

Picked this up last weekend. It popped up on Facebook Marketplace 20 minutes from my house. Two owners, sitting in the guys garage since 1991. Not stored, just parked one day and never ridden again. It's got a fair bit of mileage, and a bit rough around the edges, but its very complete and relatively unmolested. The game plan is to get her running and roadworthy while preserving the way she looks as much as possible. With the exception of that seat...

The day I got her home:

Luggage box and guard bars removed:

Left side:

Gauge cluster and DIY ignition switch relocate:

Johnie · « **Reply #1 on:** December 03, 2022, 04:01:15 PM »

Welcome to the board. You're at the right spot. The fellas here are very forthcoming with sharing tips and parts. She looks like a great project bike. Tank and covers look great. Hope the tank inside is good. Suppose the carbs were not drained. I have that same 73 I did about 10 years ago. Got mine with 5,000 miles on it. Still has the original paint and chrome. I did by new Honda mufflers as I really needed the original look. Good luck and keep us posted on your progress. The picture was taken by my daughter in law.

newday777 · « **Reply #2 on:** December 03, 2022, 04:18:24 PM »

Welcome aboard j343my and congrats on the purchase.
Are you new to the CB750 bikes?

CycleRanger · « **Reply #3 on:** December 03, 2022, 04:24:22 PM »

Howdy! Welcome!

Nice '73! Mines the same color.
Good on you for planning to keep it stock.
Looks like you've got an XS or two back there also.

j343my · « **Reply #4 on:** December 03, 2022, 05:56:18 PM »

Thanks for the warm welcome guys. This seems like a great community.

Quote from: Johnie on December 03, 2022, 04:01:15 PM

Welcome to the board. You're at the right spot. The fellas here are very forthcoming with sharing tips and parts. She looks like a great project bike. Tank and covers look great. Hope the tank inside is good. Suppose the carbs were not drained. I have that same 73 I did about 10 years ago. Got mine with 5,000 miles on it. Still has the original paint and chrome. I did by new Honda mufflers as I really needed the original look. Good luck and keep us posted on your progress. The picture was taken by my daughter in law.

The tank is pretty rusty inside and I'm sure the carbs are gunked up, but its a good starting point. That's a great looking bike. Inspiration for my project for sure.

Quote from: newday777 on December 03, 2022, 04:18:24 PM

Welcome aboard j343my and congrats on the purchase.
Are you new to the CB750 bikes?

Thanks! This is my first Honda Motorcycle.

Quote from: CycleRanger on December 03, 2022, 04:24:22 PM

Howdy! Welcome!

Nice '73! Mines the same color.
Good on you for planning to keep it stock.
Looks like you've got an XS or two back there also.

Thanks. I have a '74 and an '84 XS650 also.

newday777 · « **Reply #5 on:** December 03, 2022, 07:54:09 PM »

It's pretty amazing how doing an electrolysis cleaning take a rusty tank and make it usable again.
Here you can see the before and after electrolysis cleaning, followed up with Evaporust. The blue is 2 stroke oil to keep it from flash rusting.
BTW the 750 is a fun bike, I have a K5 that I ride.

Mark K · « **Reply #6 on:** December 04, 2022, 06:55:18 AM »

Welcome and good luck with getting your 750 back in running order. I bought mine 2 years ago on craigs list, my side covers were the same colors as your. Yours is in much better shape, so maybe won't be too bad bringing it back.

rickmoore24 · « **Reply #7 on:** December 04, 2022, 06:11:09 PM »

Nice find! Good luck with it, its a simple machine and wont take too much to get it going again.

Stev-o · « **Reply #8 on:** December 05, 2022, 07:29:37 AM »

Congrats, looks like a nice project

j343my · « **Reply #9 on:** December 11, 2022, 05:40:15 PM »

Made a bit of progress over the week.

Tank, seat, and carbs off. Moved the ignition switch back to the original location:

Float bowls look about how you'd expect after sitting with fuel in them for three decades:

Gast tank doesn't look any better:

Did a compression test: 125,140,160,145 so not bad there.

Threw a battery in, turned the key, and... nothing.

after cleaning up the fuse block and adding new fuses got a few things to work:

Horn, turn signals and signal light (solid not flashing), brake light, front running lights but no headlight/tail light.

No neutral light or oil pressure light. No gauge lights. Starter button did nothing until I pulled the clutch lever then it worked. So maybe a neutral switch issue?

Got spark also. At least on the one plug I checked.

So overall, progress is being made towards an initial startup.

CycleRanger · « **Reply #10 on:** December 11, 2022, 07:10:23 PM »

Those carbs don't look too bad at all. I've had much worse.

The neutral switch is probably stuck. You can check it when you drop the oil pan.

HondaMan · « **Reply #11 on:** December 11, 2022, 08:41:20 PM »

Quote from: newday777 on December 03, 2022, 07:54:09 PM

BTW the 750 is a fun bike, I have a BEAUTIFUL K5 that I ride.

Sorry, Newday: I just HAD to re-edit your post!

Hey, J343My: You can 'install' the POR15 kit in that tank and ride it forever. I've done several that started like yours. I did some mild rust removal (lots of ways, I used vinegar) and then the next day the POR15. It's permanent: be sure to install some screws into the petcock mount before the POR15, though, so the threads get formed OVER the screws and not IN their threads.

Did you know...the 750K3 heads were the wildest of them all, with some having 10:1 compression while others had 8:1, and the intake ports can be easily modded to improve breathing such that you get about 3-5 HP back for free? These were the last of the old sand-molded engine parts. Some even had Stellite valve guides (they were leftover and reworked K1 heads), which can last over 100k miles. The K3 often had "mixed-up" jetting in the carbs, which made them blacken sparkplugs readily. Stepping down the mainjet is their cure, especially with today's fuels.

I like rebuilding the K3 engines the most (well, next to K0 and/or sandcast) because they come roaring back with more HP in the end, just for the rebuild, than the later ones do.

grcamna2 · « **Reply #12 on:** December 11, 2022, 09:57:15 PM »

Welcome to the forum J343MY and that's an excellent CB750K3;I think the carbs look great compared to some I've seen.

newday777 · « **Reply #13 on:** December 12, 2022, 03:30:41 AM »

Quote from: HondaMan on December 11, 2022, 08:41:20 PM

Did you know...the 750K3 heads were the wildest of them all, with some having 10:1 compression while others had 8:1, and the intake ports can be easily modded to improve breathing such that you get about 3-5 HP back for free? These were the last of the old sand-molded engine parts. Some even had Stellite valve guides (they were leftover and reworked K1 heads), which can last over 100k miles. The K3 often had "mixed-up" jetting in the carbs, which made them blacken sparkplugs readily. Stepping down the mainjet is their cure, especially with today's fuels.

I like rebuilding the K3 engines the most (well, next to K0 and/or sandcast) because they come roaring back with more HP in the end, just for the rebuild, than the later ones do.

So how to tell the difference in the heads of the 10:1 to the 8:1 compression?

SanDogDewey · « **Reply #14 on:** December 12, 2022, 06:29:24 AM »

Nice find, and I love the color! Should be a fun project.

j343my · « **Reply #15 on:** December 12, 2022, 05:08:32 PM »

Quote from: CycleRanger on December 11, 2022, 07:10:23 PM

Those carbs don't look too bad at all. I've had much worse.

The neutral switch is probably stuck. You can check it when you drop the oil pan.

Yeah, they are actually cleaning up relatively easily. I will check the neutral switch, but now I'm thinking it might be on the positive side of the light. The oil light doesn't work either when I ground the wire to the pressure switch. Looking at the wiring diagram those two lights get power from the same place.

Quote from: grcamna2 on December 11, 2022, 09:57:15 PM

Welcome to the forum J343MY and that's an excellent CB750K3;I think the carbs look great compared to some I've seen.

Thank you!

Quote from: SanDogDewey on December 12, 2022, 06:29:24 AM

Nice find, and I love the color! Should be a fun project.

Thanks, I'm enjoying it so far.

newday777 · « **Reply #16 on:** December 13, 2022, 02:58:01 AM »

As to the neutral light and oil light not working, pull the bulbs and inspect the sockets for corrosion.

MauiK3 · « **Reply #17 on:** December 13, 2022, 07:04:35 AM »

Check connections in the headlight can too, they can get messed up.
Great color, very nice bike.
Vinegar is a good way to clean the tank, it's slow and doesn't go crazy on good metal.

j343my · « **Reply #18 on:** December 18, 2022, 05:20:47 PM »

Made some more progress throughout the week.

I now have functioning oil pressure and neutral lights:

The oil pressure light was two issues. The switch was bad, and the light was burnt out. For the neutral light, removing and cleaning the switch did the trick. But despite a working neutral switch, the starter will still only work with the clutch lever pulled in. I'm going to play around with the starter safety unit, but I'm thinking it may have gone bad.

Did and oil/filter change also. They both looked really good so it must have been done shortly before the bike was parked. I did discover the oil pan had been repaired at some point.

I am also three quarters of the way through cleaning up the carbs.

If all goes well I may be able to attempt a start up later in the week!

CycleRanger · « **Reply #19 on:** December 18, 2022, 05:38:57 PM »

Carbs looking nice!

newday777 · « **Reply #20 on:** December 18, 2022, 06:47:43 PM »

Great progress on it.

70CB750 · « **Reply #21 on:** December 19, 2022, 03:53:58 PM »

Welcome!

Nice project!

Stev-o · « **Reply #22 on:** December 20, 2022, 07:10:22 AM »

That float bowl looks brand new! You have some good cleaning skills...

Johnie · « **Reply #23 on:** December 20, 2022, 08:53:07 AM »

Wonderful progress! That carb looks great.

j343my · « **Reply #24 on:** December 20, 2022, 03:28:44 PM »

Thanks for all the kind words so far guys!

Also, I tracked down a seat and it arrived today:

Exactly what I was looking for. It's in great shape, but has some wear so it will match the rest of the bike. I believe its a K2 based on the pattern?

News:

Author Topic: 1973 CB750 Rescue (Read 5110 times)