'79 CB750 K

Nybz

Member
Well it is frick'n cold in the garage and I don't really feel like handling cold metal pieces right now, so I thought I might as well start a build project. Yet another CB750 DOHC! Long live!

My plan was originally to have this bike as a parts bike for my '79 CB750 LTD. Found it for a few hundred bucks, not running.


I got the LTD on the road and running great, so I started to strip down the bike, and I couldn't help but to want to rebuild this bike too. It was pretty rough, but after starting to take it apart it didn't seem like it was that bad to have all broken up in a million pieces sitting on a shelf.


So onwards and forwards!

The boat tail light is pretty rare for the CB750, I believe they were only made that way for one year. There was actually less of these made then the "Limited Edition" bike that I also have. So I wanted to keep some of the 'originalness' of the bike and not do a full blown café or anything like that. I am more of function over form kinda guy.
I wanted to create some sort of zombie apocalypse style bike. I have toned down the idea a bit but still want an aggressive looking black bike with some red accents.

Some background. I am no expert and don't have super fancy tools, but I am passionate, willing to learn, and ALMOST love working on motorbikes then actually riding them. I am a farmer, and the motorbike mechanics world gives me a chance to put my mental energy somewhere else other then the million things that need doing in the field.
I have mostly been active on the two DOHC forums and have gleaned a lot of information from some amazingly knowledge group of people. (what did people do before online forums?)

So I started buying some parts.
Randakk cycle shop full carb rebuild kit....not cheap but highly suggested by the pro's...didn't want to skimp on rebuilding the carbs
New carb rubber insulators
New coils, wires, caps and plugs
Capitan over on the cb1100f forum has made better then stock cam chain tensioners for the A chain and B chain. Not cheap! Ouch, but they are suppose to outlive the bike and dramatically increase the life of the cam chains. So bought those along with new A and B cam chains.
Spokes
wheel bearings
beefy dual sport tires
fork seals
new progressive fork springs
Front caliper rebuild with new piston
After market master cylinder
found some 900 cams...direct swap, more lift and duration...suppose to be great improvement.
clutch springs and fibers. Also got a clutch dampener kits. Which replaces the stock rubber dampeners are rock hard and cause a clutch rattle at idle. (Noticeable on my LTD)
upgraded cam holder bolts
Oil seal kit
gasket set
changed out the huge 630 chain for a new X ring 530 with new sprockets. Keeping stock gear ratios.
Steering head bearings
New shocks


Pretty much my idea is to create a brand new bike, replace any old rubber, replace any parts that are past their prime, reseal the engine, repaint, clean up every nut and bolt.

That's it for now....
 

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The Jimbonaut

Active Member
Re: '79 CB750 K

Great intro mate and a great project you're got on your hands there! I'll be following your build with much interest, particularly as some of the mods you're planning (the cam chain tensioners, the 900 cams etc) are now - having learnt about them over on the other forum you pointed me to - on my radar. Good call indeed with the Randakk's carb rebuild kit - I bought the same one for my '81 K and never had a problem.

Not sure if it's just me but I can't see the opening photos on your post, just the last 6.

Welcome, and looking forward to watching your ride shape up man
 

Nybz

Member
Re: '79 CB750 K

Well it is above 0c and I finally got my ass out to the garage.

I have done a bit off work already on the bike that I will post about later, but today is fork day!

Had some pretty bad rust on the top of the fork tubes, but not low enough to effect the seal area, thought about painting it. But I happened to find a spare set of forks from the not so local bike wreckers shop for pretty cheap.

Tubes look great just the lowers are badly painted.
Rebuilding time!
It is alot easier to take apart with them still attached to the triple tree. Made a quick bolt ‘tool’ to help me get off the top cap.

Comparing the new springs I got to the old one, they are smaller but they come with a spacer, which if installed would longer in length then original.
I wonder if I cut the spacer down to size to fit the original length or install it as is, and probably wrestle trying to put the top cap back on.
I got them from vintagecb750, and upon a closer look they say progressive ‘type’ springs. Perhaps I will send them a message.

No matter how many times I try and take out the fork seals it is always PITA! There has got to be a better way!
Tried some heat, WD40, seal remover, curved screwdriver....and yeah I protect the fork lower from damage by prying.
Anyone got a secret?


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Nybz

Member
Do these pictures work better?
I tried the do the ton app to post these pictures....had a hard time posting last night. They kept posting upside down.

How do you post pictures in the post, in between sentences, instead of just at the bottom?


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The Jimbonaut

Active Member
Dude get an Imgur account if you don't already. At least that's how I post my images. Works well, easy to use. And free. We like free. You copy the link that imgur generates (the BBCode) and then just paste it into the body of your text, wherever you want the image to show up. Job's a good'un.
 

Nybz

Member
Thanks Mr Naut. I will give it a try.

Well a simple way to get out fork seals is just to add more extension for leverage. Came out easy breezy.

It has been difficult trying to paint in a cold damp garage, but I got a system going that I think will work. Anyone got some tips beside hot water heating spray cans, and a little make shift heat booth.

I have been heating the metal before painting with a blow torch and it is amazing how much moisture is held inside the pores of the aluminum. Dripping...Good to get that out before painting I would think.


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The Jimbonaut

Active Member
Dripping? For real? I had no idea aluminium could hold that much water. I’ll be painting my engine soonish - never thought to heat the engine that much before paint. Maybe I’ll get a heat gun on mine first. Good call.


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Nybz

Member
Getting the rear hub painted and ready to get put back together.

I used primer, semi gloss caliper paint, and a chemical resistant clear coat. I could have done a better job at taping. I will clean up some lines.

I really debated powder coating parts or not. Especially rims....might regret it. It is just really expensive to take it to a local shop here, for some reason? But I am hoping this paint will do the job with proper prep.

I might pull the trigger on a home powder coating kit from Eastwood for my next project. Cheap enough and would pay for it self quick. Just need figure out 220v out to the garage to put in an oven.


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pidjones

Active Member
I'm on Tapatalk right now, so can't give the exact process, but there should be a link in the web site version of the forum that says INLINE. Make sure your cursor is where you want the photo, then click place inline (I think you do it after uploading the photo).

BTW, there is a good chance that the water you see dripping off aluminum when heated with an open flame is water condensed from the flame. Best to heat with a heat gun. If you had great results though, carry on!
 

adventurco

Nick Ol' Eye
DTT BOTM WINNER
I did my hubs like that, but painted the entire thing black and then used a paint pen to color the fins gold.

I like to keep my paint indoors if its going to be cold out, along with the part to keep it at room temp. Spray and then keep a space heater near it and should be good to go. Good progress so far though.
 

Nybz

Member
Carb rebuild using Randakk kits. Very highly recommend. All vitron o rings and includes all air cut offs, and accelerator pump.
Here is a great carb rebuild guide, such a great resource. Print it off and follow everything! Especially the part about clearing the slow air passage.

https://www.dropbox.com/s/ufw8lra6jj6fgcq/Honda_Carb_Manual_revG.pdf?dl=0

Getting the engine apart....kinda not so funny story....I wasn’t going to separate the cases, just clean up the head and new gaskets and seals.
I have this oil seal kit and I saw this counter shaft oil seal behind the front sprocket. So I am like I will just remove this and replace it. So I screw a screw into the seal to remove it.....I am screwed! The seal has a lip in it that can’t be pulled out without separating the cases!
So I ended up splitting the cases and found my main bearing were toast anyways. The copper showing is no good. It must have been oil starved or dirty at some point, cause there is only 20k km on the engine.

So I ordered all new main and rod bearings(Not f’n cheap!) and some Hondabond. There is a few o rings down there too I replaced. Everything else in the transmission is copacetic.

Had one of the case bolts snap on me after I read the wrong torque valve...careful! Lucky I just went into my local fastener shop and they matched me up a bolt to fit.

A alittle tip for reattaching the cases together, is to tie down the primary chain tensioner until the cases are together. Helps with keeping things together, cause there is little bits in there that like to jump out. Then just cut it out or untie.

Fun times!


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Nybz

Member
oh yeah....and while in the engine found some deep rust pitting in one of the cylinders.
Tried to hone it out, but it was too deep to keep within spec, so I got the cylinders rebored and got new pistons and rings from cruzinimage. 1 MM bore to 63 mm total

The 1979 CB750 had problems with the four back middle cylinder studs leaking oil from fractures in the case....so following the 1979 service bulletin put out by Honda, I took out the back studs and put high temp sealant on the threads to seal them up.

Sorry for the crap photos....in the second picture you can see the top of the new cam chain tensioner.
 

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The Jimbonaut

Active Member
Looking good - is that the tensioner from Captain (Brent)? I got in touch with him re. his overhead tensioner as my oem one is busted. Did you get his overhead tensioner too?
 

Nybz

Member
I didn’t get the B chain tensioner..... It can be removed without removing the engine for the frame so i opted to not get it, as my B tensioner still looked good. But yeah replace it with something better if it is busted.

The A chain takes more force and is a case splitter, so I took care of that the best I could.

On 1979 models the B chain GUIDE is alittle bit more weaker then the rest of the DOHC line. Mine had a broken leg attaching to the cam holders. Must have been a loud rattle when running, I wouldn’t know.
But I got it replaced with a later model DOHC, which is stronger.

You can see in the picture 900 cams, new high quality A and B cam chains(M), upgraded cam holder bolts and the ‘new old’ b chain guide that has to be bent to fit the new A chain tensioner. I heated up the copper washers under the cylinder head nuts to help them seal (anele?) , and added a bit of case sealer as back up.


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cxman

Active Member
make sure while you are in there you remove inspect and loctite and torque the three starter clutch bolts just behind the advancer
they are notorious on all the dohcs for working loose and causing a "rod knock"

if your bolts have been rattling put in new ones
 

The Jimbonaut

Active Member
I can absolutely attest to that - had a loud knocking from the side of the engine on my other 750 which turned out to be loose starter clutch bolts. Loctite and torque and job’s a good’un.


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Nybz

Member
Done and done thanks

I do have a starter clutch rebuild kit as well that I will wait to install depending on what bike needs it. My LTD just spins sometimes without engaging, so will probably be that one.


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cxman

Active Member
if its a aftermarket rebuild kit use honda springs with it

aftermarket ones fail in weeks
 

The Jimbonaut

Active Member
Hey man glad you posted that image of your cylinder head. Reading the Clymers while I was dismantling mine it mentioned something about two rubber caps on two cylinder bolts which, in my cylinder head, weren't there. These things -



I can't find them on any parts fiche, and I'm wondering what they actually do?
 
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