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After 10 years, this 1982 CB750 SC Nighthawk is running again - Intro/Progress

deniv

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Hey everyone I have made a few posts here and there but I haven't properly introduced myself. I live in the Austin, TX area and recently acquired a 1982 CB750sc Nighthawk for $400. It was in pretty rough shape, but I am stubborn and determined to clean her up and put her back into service. Although I've been riding bikes belonging to friends and family off and on for the past 10 years, I've never had one of my own or worked on one. Besides a doing a full brake job and modifying a condenser fan to replace a dead one, I don’t have a lot of experience working on my truck either besides easy things I can just swap out.

I knew going into this that it was going to be a ton of work and that I would end up spending more on parts/repairs than I did on the bike itself, but I wanted something I can be proud of. I recently got married and we are waiting a while before having kids so I am also trying to take advantage of the free time I have left before going down that path.

Now for the build so far: I have been bad about taking pictures and haven’t managed to take ones of every little thing, but it’s better than nothing.

My two goals in order of priority: 1. Get it running by focusing on repairing one system at a time 2. Get it drive-able with the same focus on systems as #1

Here is the bike the day I got it (just over 1 month ago):
Day 1 left.jpg Day 1 rear.jpg Day 1 right.jpg

What I’ve done so far

Fuel System:

The master cylinder was broken and the brake lever bent. Clearly the fluid spilled all over and the PO never cleaned it up because there was gooey black crap was all over the tank and after wiping it off I noticed that the paint was eaten away. In addition everything is super dirty and in need of a lot of TLC.
The brake fluid appears to have ruined the tank. When I went to remove the right side emblem, the metal almost gave in. No way was that getting patched. The inside also looked disgusting.

Ruined tank0.jpg Ruined tank.jpg Ruined tank2.jpg

Fortunately I found a suitable replacement on eBay somehow for $70 with shipping. The petcock valve even appears to be in great shape.

New tank.jpg

I started restoring the tank by dipping the entire thing in a trash can electrolysis tank for about 3 days. I didn’t bother fitting an anode on the inside of the tank. Part of me sort of wishes I had because it would be cleaner, but I wanted to seal the inside with Caswell anyway and it allegedly bonds well to surface rust. That said, my method of dipping the whole thing managed to remove a lot of the rust inside.

Electrolysis 1.jpg Electrolysis 2.jpg

After it came out of the tank, surface rust appeared immediately. Not a big deal though because I still needed to strip whatever paint remained after the electrolysis bath using aircraft remover. The worst part was getting the decals off. A heat gun and a razor blade made it slow, but doable.
After stripping the paint, sanding the small amount of surface rust that appeared, and bodyfilling some small dents, it was time to paint. I have been taking my time with it and still need to wet-sand and then clearcoat it, but this should give you a good idea of how it will look.

primed.jpg painted.jpg

Next I pulled the carbs and airbox which was a major pain in the ass to do for a first-timer. The carb boots to the engine and to the airbox were hardened and may as well have been vacuum cleaner attachments so I ordered a set of brand new ones.
The airbox was pretty bad. It wasn’t as bad as some of the pictures I have seen of people finding rats nests, but I did find this little guy.

cockroach.jpg

I cleaned the airbox out thoroughly and replaced the filter. My wife wasn't very pleased one day when she came home to it soaking in our bathtub.

Next I went to work on the carbs. I’ve always heard people complain about having to clean their carburetor so I thought it was some terrible process, but once I figured it out I really didn’t think it was too terrible. The worst part was taking them apart. The corrosion in them made it very difficult.
Once they were apart I found that the process of actually cleaning them was almost relaxing.

I replaced all of the components with a rebuild kit from 4into1 and reassembled them using brand new stainless steel hex bolts. I also replaced all of the air-cut offs and the accelerator pump.

This carb rebuild guide I found was a Godsend. I followed it line by line as a newbie and was successful. I highly recommend it to anyone with a similar set of carbs. http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revG.pdf

Apparently I can't attach any more photos so I will continue this post as a reply
 
Clean vs dirty carb pictures:
Dirty carb.jpgclean carbs.jpgcarb bowl comparison.jpgassembling carbs.jpg

Before installing the carburetors back onto the bike, I took the opportunity to replace the choke cable, but I was too excited about how close I was to getting it running again, that I neglected to install the throttle cables.
As much as I hated removing the carbs from the bike, putting them back on and then getting the airbox to seat on them was even worse. After a few beers over the course of a couple hours of frustration in the humid 90 degree weather we have been having, I finally got them back on the bike.

Ignition System:

I like being thorough. While I may not have needed to do everything here, I figured I may as well in order to preclude easy things from any future troubleshooting.

I replaced the ignition coils, wires, electrodes, and plugs. Before replacing the plugs I also changed the oil & filter, dropped a teaspoon of oil in each cylinder, and turned the engine over by hand to ensure it was well lubricated after sitting for so many years.

First run in a decade

After 10 years, a repaired fuel system, and fresh ignition system, it was time to start her up.
Amazingly, it started up pretty easily. After adjusting the high idle screw it still appeared to idle a little rough, but I’m definitely no expert. I forgot to attach the clamps on the airbox boots so I assume that was part of the problem because once I put them on, it seemed to get better. Carb #3 I noticed was also pissing gas…. Lovely.

Video of the first run: https://youtu.be/f6QsRTonV3A

I pulled the carbs again and found that there was some crud in the float valve seat that I must have missed. A squirt of carb cleaner and a q-tip did the trick. After putting the carbs back on the bike (I remembered the throttle cables this time) it seemed to run much smoother and without any issues!

Drive Chain

I replaced the front and rear sprockets after finally removing a rounded bolt in a really awkward spot on the sprocket cover.

sprocket cover bolt rounded.jpg sprocket cover bolt bad spot.jpg sprocket cover bolt removed.jpg

There was a ton of crud built up near the drive sprocket, I really hope that it’s just old chain grease and dirt built up and not something else. While I was in there I also replaced the gearshift oil seal and the drive sprocket o-ring.
I have a new chain for it, but I am waiting to replace it until after I get my tires installed.

To do list:

• Tune the carburetors
• Clear coat the gas tank
• Paint the side covers and cowl to match
• Paint the emblems for the tank and side covers silver
• Clean the wheels up and install the tires once they arrive
• Install the chain
• Overhaul front brake system
o New Master Cylinder
o Used rotors measuring 5.10mm thick
o New pads
o Goodridge Stainless steel lines (retaining original design of 1 line -> Splitter -> 2 lines to each caliper)
o New banjo bolts, banjo fittings, and washers
o Goodridge Speed bleeder screws​
• Replace the fork oil – The fork feels spongy, should I be concerned about this? Not sure if this means I need to rebuild it or not.
• Grease the swing arm
• The 3&4 exhaust pipes have small holes rusted through them. I plan to patch it using a piece of aluminum, some epoxy, and clamps. This is intended
to be a temporary fix until I purchase a 4 into 1 system. I would like to drive it around for a while before spending that much on one thing.

Am I on the right track? Is there anything I am missing that I should inspect/replace/lube before I ride it?

There’s not a lot to take a picture of right now, but I’ll try to update this thread with photos as I continue to make progress.
Thanks!
 
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