Topic: 78 CB750K project, first motorcycle

[thebluemax]

Picked up a 78 CB750K about a month ago. Non running, all original, 4288 miles according to the odometer. No key. Had been sitting since 1984. Came with sales receipt, owners manual, title, insurance card from 1984, all in an envelope under the seat. Original Bridgestone tires (dry rotted but tubes hold air). Unfortunately the bike spent all it’s life in a saltwater climate which ate some of the metal, mostly just the chrome and wheel spokes.

Not sure where this project will go. The bike isn’t a restoration candidate since it’s to far gone. I’m thinking restomod.
Needs a boat load of work

Can I say that I love the recessed gas tank cap with the door and the smooth line?

[SOHC4 Cafe Racer Fan]

Welcome and good luck. Feel free to ask any questions and post your progress.

[thebluemax]

How do I resize a picture to post on here?

[SOHC4 Cafe Racer Fan]

After 10 posts, you can post photos on the forum.

Use either the attachment function or a hosting site like flcker

[thebluemax]

Electrical 

[thebluemax]

[70CB750]

Following

[thebluemax]

All of the above.  >:(Replaced key, initially had blinkers and oil light but not taillight , headlight, or ignition. Checked fuses and found blown taillight fuse , replaced fuse . Fuse blew again and now I get nothing. I’ve got the battery cables clipped to my truck battery for testing since current motorcycle battery is dead. Haven’t had it tested yet to see if it has a dead cell or is still usable.

Pretty much learning everything as I go.

[thebluemax]

The original key was lost, so I had to buy a replacement emgo ignition switch.

[70CB750]

Dont rush it.  Try to turn it over using the kickstart.  If it turns over, drop the oil pan and see whats in there.  Just  make sure it is mechanically solid before you try to start it up. 

[thebluemax]

I know it turns freely with kick start and has 150psi compression across all four cylinders. But haven’t gotten any farther on that. Don’t have that much mechanical knowledge on motorcycles, I tore down,modified, and rebuilt an 80’s Chevy 4x4. Hope to get it all worked out.

[70CB750]

I know it turns freely with kick start and has 150psi compression across all four cylinders. But haven’t gotten any farther on that. Don’t have that much mechanical knowledge on motorcycles, I tore down,modified, and rebuilt an 80’s Chevy 4x4. Hope to get it all worked out.

Very good, you will be fine.    Checking the oil pan is easy and assurance that there is nothing it should not be there.  Some people do not bother with it but I do it on new to me bikes.

Electrical issue, it just takes a ohmmeter, voltmeter, schema, six pack and few hours of time. 

The wiring on 750 did not change much,  check this out:

http://oldmanhonda.com/MC/wiring750K1.html

[70CB750]

I think this is more accurate for a wiring diagram-
http://oldmanhonda.com/MC/WiringDiagrams/CB750(K8).jpg

no doubts 

posted that one for its educational qualities

[thebluemax]

Holy rust Batman

[70CB750]

Not bad at all 

[thebluemax]

Ohm’d out some of my wiring last night. Tail light and head light have continuity. Ignition does not. Also started making a parts list .
So far the bike needs:
Master cylinder rebuild at minimum/ replacement
Front caliper rebuild
Front brake line
Front brake pads
Possibly the front rotor

Probably chain and sprocket

Shocks and hardware

Petcock
Gas tank cleaning
Fuel lines
Carbs cleaned
Air filter
Carb linkage/cable


Tires front and rear
Rims and spikes are pretty rusty so probably need full replacement

Wiring issues resolved

Oil and filter

Kill switch/start switch replaced


Should I just tear down the bike and rebuild from the ground up?part it out , raise enough money to start  over with a running bike?

[70CB750]

Dorothy, my Blue bike, came to me in similar state.  I decided to get her on the road first, do upgrades and improvements and rebuild her later.  It took 5 years and 25 000 miles, now she is boxed up and waiting to be resurrected.  In the mean time I built Red, I just did not want to be without motorcycle anymore.

That said.  I would get her rebuild piece by piece starting with tires and brakes while keeping her on the road. Lot of people jump in head first, take the bike apart and loose steam in a year or two and end up selling it as a basket case.

My$0.02 only, of course

[thebluemax]

Would a 750F wiring harness fit a 750K ? I’m looking for a new harness

[thebluemax]

Picked up Mac 4-1 exhaust last night from Craigslist. Started installing today before work. Will post pictures later.

Last night while on a plumbing service call started talking with the customer about bikes and turns out he used to have a SOHC 500 , followed by a DOHC 750 and numerous Norton’s,BSA, Triumph, and Harley’s. Long story short he told me he would help me with my project.

[thebluemax]

Bought new copper gaskets and installed them. The collars that bolts to the head....are they supposed to be oriented one way or another?

[thebluemax]

Got it bolted up last night and it seems really loose. Put the round side of copper gasket facing engine and flat side of copper gasket facing header. The little thin metal rings  that go around header with the flanged part against the header,


Got the headers bolted on last night. Looks sweet. Now onto everything else.

[thebluemax]

Took some scrap brass to get money towards the project.

Changed spark plugs, changed oil and filter, mocked up exhaust, bought some goodies ( new grips, new air filter, new and battery, new master cylinder, new start/kill switch assembly new throttle tube) also picked up a used copy of SOHC clymer owners manual.

Still chasing down electrical. Probably going to buy a new starter solenoid, new wiring harness, new rectifier/regulator, electronic ignition, new coils, new plug wires, new bars (since everything on them is getting replaced) also new cables for both push/pull, new choke cable, new brake lines .

Plus tires , rims,spokes,chain  sprockets.

The work never ends. Trying to make the bike as safe and reliable as I can. It’s my first motorcycle period, do I want it to be right.

[minimo]

100% with calj here.
You can try simplifying your set up by eliminating/disconnecting some or all of the lights - at least the headlight, which in park mode can be a total power suck. Study the wiring diagram and familiarize yourself with what the colors correspond with on the bike. Clean your contacts - Dremel and a brass wire brush wheel are your friends. Also, chances are that you are trying to start the bike via push-button start. These start buttons are notorious for crumbling up inside and going bad (kinda like my own conscience).
Before you go spending all that coin, open up that throttle/switch cover and take your time to clean it all up, buff up the contacts, check to see if those electrical and plastic components are all intact, put it all back together and then give it another shot. 
But like the others have said, you should be using kickstart instead.
Your key ignition also sounds suspect. I’d check on those connections - clean them all up and maybe even open it up and clean up the internals. Depending on how bad the key ignition looks, it may be time to replace it. There are decent replacements available (via 4into1.com that I’ve experienced). The connector base on the old ones especially the plastic tabs that keep them in the metal housing can disintegrate and with that the contacts become loose and it’s all garbage. If you’re not getting a nice “click” between key turns I’d look into replacing the ignition switch.
Most electrical issues that I’ve dealt with and have seen/heard others struggle, the problem has usually been right under my or their noses. Check everything and then check it again. Don’t throw money at the problem right away, as other have said, that may not solve the problem. And it’s great that you take note of everything. When you’re chasing electrical issues, it’s usually best to be systematic about it and make a single change; not make multiple changes at once and pull the trigger. Also, wire colors fade and I’ve mistaken whites for yellows greens for blues sometimes (I’ve even mistaken + and - to the battery, no bueno!) and I’ve learned since to give a thorough wipe down to expose what’s really under the 40-odd years of crud. Take your time.


Sent from my iPhone using Tapatalk

[thebluemax]

Replaced battery and charged it. Checked all the connections, (currently have the turn signals and head light disconnected) . Turned the key and low and behold. We have light! Oil light, neutral light, and instrument lights. Have a new start/kill switch and cables on the way. When they come in I will see if it starts.

Also working on vacuum and fuel lines.

*edit* jump started bike and starter spins but no spark at plug wires.

[thebluemax]

So as of tonight I have no spark at the plugs but power on black and white wire to coils from engine stop switch. Have power at blue and yellow wires on points using a test light.

What gives? Next connection or component to test?