"Poor Kid" 1981 CB750K - first build

kraptastic k

ask me how i got here
Hello all.

I bought an 81 CB750 from a guy in Dallas, TX as my first bike. This occurred in 2011, I believe December, when I was waiting tables at a sushi restaurant and living in a different county. Cost me $600 initially. Here are a few flattering shots from PO (we're now friends after the trade) and some less flattering ones as I rip it apart.

Note: This bike was in rideable, fixable shape- but for some reason, I couldn't leave well enough alone. Maybe perfectionism?
 

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I started checking things out. The exhaust was rusty, but possibly servicable. The seat, which had been swapped from a smaller Honda bike of a different year, looked oddly perfect. I loved the classic lines of the bike.

The bike had only two PO's, but it had taken an apparent beating... I started developing a plan to fully remake it in a more desirable image.
As I had yet to really get into the scene and become self-aware, I was setting myself up for cafe 'ricer' status... worried about cheap filters and stupid bars and shit. Just copying what I saw in photos on google for "cafe motorcycle..." ugh!

Unfortunately, I was too proud to ride around on a semi-stock rust rocket. I pulled some other things, such as broken bar controls, fried ignition units, and a loosely dangling front brake master cylinder that read "Kawasaki."
Also, the choke cable was hanging out the left side of the carbs, duct-taped around a hose clamp as a makeshift handle. And it wouldn't cold start. And the tank had a huge fuckin' hole in it. As I worked through the initial phases of this plan, I saw more and more that I could not bear to leave alone.

Talk about getting in too deep... I had never even ridden a road bike before! I didn't know what a frickin' carburetor was when I purchased this bike. I wasn't certain of doing any of the "fixing" just yet. But I knew I could do the research and figure it out, because I've seen dumber people do cooler shit. So, over the course of about a year, I began accumulating parts. A ton of them, some OE replacements and some universal. I was applying more of the "bought" in "built not bought," and it would come to bite me right in the ass. During this phase, I did pick up my very rudimentary tools, which I'm still adding to. Didn't have much of a dad around for this shit.

I probably have spent about $2500 on the bike to this very day, when tools are considered as well. It still hasn't run.
 

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So, I moved from apartment, to house, to yet another house with this bike, and all the boxes of parts I had accumulated. What an idiot I must have looked! But at this third dwelling I had made a change- a damn garage. Getting a garage has helped, but also job changes have permitted better income. Buying tools was no longer an object, and I committed to stop making excuses.

I took the front forks off to be rebuilt. I entrusted the job to a local shop. The place was sketchy and I'll never go there again- dude kept my forks for 6 months, giving me excuses about not being able to source some of the rings inside them. I found the ring, took my forks and my damn money home and did it like a big boy.

It wasn't a common job and I'll happily offer my advice to future DOHC builders on that one. I also had new tires mounted and balanced. I rebuilt the steering head and placed new bearings in it and in both wheels. Nice and greasy.

Knowing that most or all of the charging system was fried and mangled, I ripped every component out. Roughly $600 went to replacing the system. I was stuck on that bastard bolt as we began to call it- the alternator rotor bolt. I began work on it after the engine was out of the frame, which is apparently the worst time to jack with it. What a triumphant day when it came off! Unfortunately, a large chip in the case occurred before we got it off.

What a traumatic event... I was super pissed. A few days calmed me down and I got the epoxy out. Good thing this part of the case doesn't hold oil :( but it's a nice snug fit now.
 

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Ooof... just looked at the above pics. Need to get a real camera instead of a phone.

Anyway, so as space and resources were available, I began to move forward slightly. This was around October 2012 when momentum picked up slightly. I had pulled the engine out of the frame and discovered the suspension was totally dead in the back- the struts were as deflated as a 70 year old Kardashian ass.

I had an explosive moment when I went through the carbs and found that I could not confidently continue with them. The idea of being bound to that tacky ass airbox for the remainder of the bike's life was too low a ceiling. I gave the carbs and some rebuild kits to the PO of the bike for his CM400. Then, I bought some Mikuni BST36SS carbs, the ones from the GSXR-750s. As if I wasn't already lost in unfamiliar territory...

I painted up some engine cases after inspecting under the valve covers for rust. Oil was coating every surface and the gasket was in good shape. No reason it shouldn't work the same as it did when I took it out. Thank god I know how to operate a can of paint, I thought, then the shit chipped. And then I remembered it's 4 bucks a can, oh well! Only pissed me off because I did a 4 day process of multiple clear and color coats.

Further inspection under the front sprocket cover showed a ton of dirt/oil buildup and a severed wire. I forgot what it does, but I did a solid job soldering that punk back together. No room for error. I cleaned the crap out of that area to try and locate a possible oil leak, but no dice- the hull is intact cap'n. Unfortunately, the pulse generator cover appears to have been epoxied. It's not well done (by my standards, which are very f**kin high) and I'll have to redo it better.
 

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Thanks to Rich Ard for the ignition units above. You rock, Rich.

I cleaned the frame up slightly and covered some rust spots with (you guessed it) flat black. New suspension went on and I got wheels back from mount/balance. There's the stance... a sign of age I'm certain. Turning a cruiser into a streetfighter. But I ordered that suspension specifically for two up. And the seat looks like a brat seat. Hmm. Two- up Bratfighter? I don't know.

The carb swap is nearly complete. Yellow 1/4" fuel line for the vents and 5/16" fuel line. I think I about nailed the boots but the hose clamps are far too long. K&N filters and Dynojet 136 mains are in place with all the Dynojet stage 3 shit installed. I hope the tuning is easy...

I put the exhaust on as well. Why not, right? Looks clean! I put the busted tank on for a mock yet again. Keeping me motivated. Unfortunately, I now wait on properly sized cables before I can install the air filters. That's what I get for ordering universal.

A new tank came in the mail a while back, I had previously treated it with The Works, but about a month with gas and Sta-Bil in it and I think it may need another rinse with acid. I know I'm not gonna be able to mount the tank and seat like this until I see a welder. I know a guy of good repute who I plan to have make me a whole subframe/rear tail light.

Unfortunately, I do not yet possess that level of expertise, garage builder elitists >:)

I am considering multiple options for tank styling. I am heavily influenced by MotoHangar's recent CB750 and I would like to try a brushed metal finish with clear coat. Does anybody have input on the process? I can furnish sample pics.
 

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To continue my meandering discourse from above:
I still would like to build this bike completely, all the way down to custom engine internals and tuning- however, as I am building skills, I want to get this bike back to running, legal status before all else. Understand my goal, gentlemen! Then hopefully I can build better with riding experience at hand.

As the Poor Kid currently stands, it is almost 100% road ready, with a few small caveats. I want it to remain slightly unfinished in case I drop the fegger on my first day. I hope you understand, crashing a perfectly polished bike would crush my heart. That's not to say I won't spraypaint or grind something just a bit for learning's sake. All things can be recreated with the right tools and experience. I think that's the lesson I've picked up here.

This week's Problem list:

I cannot source the front brake piston, as multiple orders and searches have brought me pain and a ton of returned shipping fees. When this is done, I can finally place fluid in the brake system.

I am working to finish the wiring, having repinned and soldered a good deal of the bar controls to obtain a clean(er) look. But as has happened before: I am confused by universal parts. This time, I am trying to wire the universal killswitch, which does not have a pinned connector. Also, the OE replacement reg/rec has one fewer wire. The present wires are pinned in a different configuration. This doesn't match the diagrams I have, building both headache and frustration faster than motorcycle. I suppose I will figure it out in the next days.

Throttle and speedo cables: too short/too long. Universal throttle tube wasn't compatible, OE cables were too long for clip on bars, etc. The usual. Handling it through Swifty's Motorcycle on Camp Bowie... What a great shop. Terrific service and absolutely competitive pricing. Support the locals! After this I can install air filters.

Everything else is totally finished. I'm still kind of in disbelief that I did this much, but I owe a great deal to this website and my friends. My standards are high, but I really need to see some critical commentary from other builders- I'm isolated from this community mostly by location.

Feel free to ask for other pictures if you can help with the above... otherwise, check back after I get paid hahah!

Thanks,

krap
 

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Good to see you're getting there K - keep at it man.
 
That's terrible criticism Rich! Thanks for the support man. A matter of time now, hopefully only weeks. Damn wiring will be the death of me

1981 CB750K (getting there now)
 
Keep at it man, it looks like your on the right track and your fixing your problems properly. All the frustration will be worth it once you start riding it.
 
I keep hearing that. Hopefully I'll get to experience it soon! Figured out some stuff on the wiring with help from a friend and ready to solder tomorrow. Diagrams are making my head spin tonight.

1981 CB750K (getting there now)
 
Some more chatting with Eric at Swifty's and a bit of ingenuity have resulted in filtration installation. A bit more soldering tomorrow... Only right hand bar control and reg rec install remain before tuning and riding.


Notice I had to grind/bend the tabs that used to hold side covers. I'll compromise, this is a mock tank and form must follow function.


1981 CB750K (getting there now)
 

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I decided that this whole brake caliper nonsense had to end. I put the old piston back in. the piston won't be pushed out to the pitted area until the new pads wear down, giving me some time to ride and consider upgrade options.

Still confused on the reg rec- we think we have it figured out, but I still feel wary about some of the wire color choices and pin sequences. If the battery dies, I guess I'll know. Spark wires and coils are soldered and run as well. I couldn't figure out how to run my own kill switch, so I nabbed one from Rich Ard.

Discovered a nice oil leak out of the left crank cover. Looks like it was dropped once or twice. If anyone has a good one of these I'll buy it off them - just a PM and a price for anyone reading.

To my horror, the tank I de-rusted and stored with fuel and Sta-Bil (cause of red coloring in fuel) is of course rusted again. Dull brown rust on the bottom, no shiny metal on the inside.
Damn, that is irritating. I mean... Really irritating.

I could be riding on Friday, had I not trusted one of the billions of half assed rust removal quick fixes.

What the hell do I need to do to kill and stop the return of this rust? It's time to ride!

...anybody?
 

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Hmm. Vinegar and mineral spirits, another one from the forums. I have all that stuff as it is. I'll try it I guess. Will post results as I bite my fingernails and stare at the mailbox waiting for my switch. Oh, and payday is tomorrow. It's like two Christmas' occurring simultaneously.

1981 CB750K (anxiety destroyer)
 
Vinegar does the trick - then, afterwards, rinse with a baking soda solution to neutralize the acid, and then coat liberally with oil (I use Marvel Mystery Oil, because it's fun - some people use WD-40 or two-stroke oil or whatever's handy).

Your switch oughta be there tomorrow - best luck man!
 
I know man. I'm still impatient.


Will the added oil disturb my carbs or the running of the bike? I plan on running it shortly after de-rusting.

1981 CB750K ()
 
Don't pour a whole gallon in there or anything, just enough to coat the thing. :)
 
I really want to make sure this thing is well coated. Can't I just put fuel in it directly after, if I use the mineral spirits to dry it out? Of course it just rained in TX so I may be forced to wait for humidity to decrease.

1981 CB750K (getting there now)
 
I went into the garage with low hopes today. Got back from the hardware store and grabbed my switch out of the mail.

I wired the switch and mounted it. All the headlight functions are present and the on/kill and starter features all worked. Everything seemed cool. Secured almost everything in the wiring.

I hooked up my tank to a car fuel filter and ran fuel to the carbs. Oops! The 'On' position causes fuel to leak out from under the petcock. 'Res' seemed to work, and fuel flowed into the carbs. But then I realized that fuel was overflowing... Not from the vent tubes. But from the air filters. No throttle- just fuel through the line into the cleanest and driest carbs you ever saw. I think it may have been the petcock, as the GSXR carbs have a vacuum line that runs to their petcock, and the Honda petcock lacks a second port. Well... I had to kill the tank rust anyway so I'll have to find a suitable petcock at the same time.

I removed the tank and just held a long piece of line up to feed fuel into the engine. With everything plugged in, I tried to start the bike twice. Saw some blue smoke out of the 3rd cylinder where the header pipe meets the block. Was told to check "split keepers?" I'm not really sure, but I'll check tomorrow. Below is the kit I received and I may have installed the little flanged pieces in the lower middle of the pic.

My seven wire SOHC reg rec got very hot initially (enough to make a smell of burnt plastic) perhaps because I used it on a DOHC... I am determined to make it work and will have to run tests with the ohm meter...

Set the front brake up as well, but we had trouble with air in the lines and decided to leave it overnight to gravity bleed.

Itching to just hear an idle now.



1981 CB750K (working out kinks)
 

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Goood morning.

My buddy dropped by and wanted to see some progress, so I went through the steps of setting up the tank and fuel filter again to try and start it. Plugged everything up with a big car fuel filter and ran some fuel in, pulled on the choke and hit the starter.

I was surprised by a strong, loud idle after the first press, no exhaust leak, and when I reached back to touch the reg/rec, it was still room temperature. I killed it before the fuel ran out of the line and stepped back, blown away.

I can't believe I got those swapped carbs to work so well on my second ignition attempt.

Of course I'll be messing a little with the carbs soon, but this is the list for the next few weeks:

New petcock with an 8mm vacuum hose barb or fitting on it.

Continue bleeding front brake: lever feels soft and bike will not stop when lever depressed. (Cheer up lever)

Put vinegar in the gas tank for two days and annihilate rust, follow with baking soda/ water, then dry with spirits, grind side cover tabs off tank and fill with fuel/replace onto frame...

Ride?



I don't even have a helmet!



1981 CB750K (on the homestretch)
 
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