Topic: "New to me" 77 CB750K, have some questions about it.

[Mattydiah]

Hey everyone, I just bought a 77 CB750K.  The bike is in pretty good shape, needs some paint, a carb rebuild, other maintenance.  Anyways, I'm just starting to tear into it, and I have some questions.

1)  I took the tank off, and there is a little nipple that comes out of the bottom of the tank, towards the middle.  It didnt appear to be hooked up to anything.  Is this just a breather that I can run to the bottom of the bike, or does it go to something specific?

2)  The throttle is obviously a pull-pull setup, and someone converted it to push-pull (i.e. only the front cable is hooked up), was this a common mod back in the day, or could something more be wrong with it?

3)  Are there still parts availiable to rebuild the caliper on this bike?  It appears to be frozen.  The master cylinder looks good yet though.


Also, I came across some Remus exhaust pipes off a Ducati Monster.  Since this bike has no exhaust, I figured I could pick up a couple of 2-into-1 drag pipes, and cut and weld them to the Duc pipes.  Anyone have much experience with mating modern sportbike pipes to our old headers?

Here is a picture of what I'm dealing with, including the Duc pipes mocked up to it.

[754]

1) probably your gas cap filler overflow, if you spill a bit or overfill  slightly it will run down that to a tube that should be hooked up to it and dump hopefully away from your rear tire.

2) does your kill swich work, if it does and throttle seems OK, you can probably leave it

[BobbyR]

Hey everyone, I just bought a 77 CB750K.  The bike is in pretty good shape, needs some paint, a carb rebuild, other maintenance.  Anyways, I'm just starting to tear into it, and I have some questions.

1)  I took the tank off, and there is a little nipple that comes out of the bottom of the tank, towards the middle.  It didnt appear to be hooked up to anything.  Is this just a breather that I can run to the bottom of the bike, or does it go to something specific?

2)  The throttle is obviously a pull-pull setup, and someone converted it to push-pull (i.e. only the front cable is hooked up), was this a common mod back in the day, or could something more be wrong with it?

3)  Are there still parts availiable to rebuild the caliper on this bike?  It appears to be frozen.  The master cylinder looks good yet though.


Also, I came across some Remus exhaust pipes off a Ducati Monster.  Since this bike has no exhaust, I figured I could pick up a couple of 2-into-1 drag pipes, and cut and weld them to the Duc pipes.  Anyone have much experience with mating modern sportbike pipes to our old headers?

Here is a picture of what I'm dealing with, including the Duc pipes mocked up to it.

You will want to restore the push pull cables. When you make brake the grip moves forward which forces the slides down further than the spring will. For example, I can take the idle down from the 1200 normal idle down below 1000 just by twisting the grip forward. When I am running the choke a 3K rather than clunk it into 1st, I just twist the grip, hit the shifter and go. Honda knew what they were doing.

[shoemanII]

the cam chain tensioner has bolt/lock nut, nothing special.  can measure the one on a spare if you'd like.     
bobp 

[upperlake04]

Anyone know where I can get a new AP plunger?

https://www.partsnmore.com/cat_index.php?model=cb750_sohc&category=carb

[754]

I did not answer to the cam tensioner Q, as I think it is not a normal bolt.. I think the end is turned down past the thread & the end faced.

I have one in the shelf, made a note to check it today, and the part # in the book.

 But as long as the end is ground perpendicular a bolt shoiuld work.. will check the length today.

[TwoTired]

Anyone know where I can get a new AP plunger? 

You can still get this assembly through the Honda dealer.  I just did a couple months ago.  16021-393-004  (assuming it is the same as my 77 750F)  This includes replacing the base of the pump with it's included check valve.  There is also a check valve in the carb body that must function, too.  Check it now or risk pulling the carbs again to get the pump working properly.

If I spray carb cleaner into the hole where the accelerator pump pushes fuel, I get a fine mist out of all 4 carbs, not 4 streams of carb cleaner, like I would have expected.  There is some serious resistance in there.  The full can of carb cleaner still had a problem pushing through.  Shouldnt those be more open?

I can't tell for sure what you are refering to.  But, the carb delivers fuel to the pump body through a check valve.  It should only allow fuel from the carb bowl to the pump and not the reverse direction.  The other hole feeds the posts in the carb throat.  This is where the fuel is shot into the carb throats.  These are pretty tiny streams when the pump is working.  They will spray mist if you feed them with aerated liquid.

ALSO, there appears to be a relay in the headlight bucket that is activated with the starter switch, is this at all correct?  I mean I would think there is a starter relay, but up in the headlight bucket?  Seems more reasonable to have it back under the sidecover with the rest of the electrics.  I am getting a strong feeling that some of the electrics are hacked together.

There are no relays in the headlight bucket shown on the stock wiring diagram.  Welcome to undocumented previous owner modifications.  You've another puzzle to figure out.

Cheers,

[TwoTired]

Is the check valve that little square piece of plastic in the pump bowl?

No. The square plastic holds the ball captive.  I've never removed it.  I kept cleaning/flushing till the ball would hold pressure in one direction of flow.  There's one in the float bowl body as well as the pump bowl body, too.

Cheers,

[TwoTired]

Your right side switch is from an earlier model.  The 77 did not have an option to turn off the headlight.

The 77 starter button itself disconnected the headlight when pushed and engaged the starter.  The older models did not do this.  Your headlight relay may have been the way this function was restored when using the older starter button (defeating headlight power when the button is pushed.)  I decided to do something similar when I can no longer rebuild my button switches. Old buttons were momentary single pole single throw (two contacts). The new buttons were momentary single pole double throw (three contacts).

Not sure you really need a whole new harness based on what you have told us so far.  I would think it would be easier to repair the one you have.  But, it will require some thought.


P.S.  Get yourself some #0000 steel wool and rub that chrome with it.  You'll be glad you did.

Cheers,

[754]

Cam tensioner bolt does have the end turned and squared.

Part # 90009-300-000...up to # 1044805 & earlier
Part # 90009-300-010....after # 1044806...(includes 78 I think)

You can prep the end of the bolt if you are careful, but if you dont do it, it may slip..it is 6mm x1.0 x 2mm long.

(didnt check pitch, but like living dangerouslly!)

May have a complete harness that has been  stored indoors since around 1980.. anyone got part # ?

[TwoTired]

I just noticed.  There are a lot of front end bits that are not 77 Style.  The advisory light block, the fork accordians, and the fender look like earlier style.  May have an earlier fork assembly grafted to the frame..   What makes you think it is a 77?  Does the VIN plate say so?  If so, then the grafted front end.

But, because of all the cross polinated parts, a new harness most likely won't be a direct install.

Cheers,

[754]

I know the that I have a harness or two hanging on a rack, and at least one has the number tag on it.

But I am not sure I can visually tell the difference between  73 & 77 harness.. there should be some difference..

[TwoTired]

I believe the the 77-78 has block connectors for the bar controls.  Main Harness would have these to interface.

Chase the bundles for your current bar controls and see if either has a block connector interconnect instead of the individual bullet style.

Cheers,

[eurban]

The stock harness for the 77/78k bikes has a combination of block connectors and bullet connectors in the headlight bucket.

[mick750F]

You should be able to find a wiring diagram in one of the links in the FAQs. The part # for an F2/3 harness is an F2/3 specific #. Not sure what would make it different than the K...

Mike

[TwoTired]

There is a wire diagram in the Honda CB750 shop Manual pg 264 for the K7

There are no Bullet connectors Right side bar control.
Right control block connectors Wire colors:
Y/R
Bk/R
Bk
Bk/W

Left control Block connector
Br/R
LB
O
Gr
LG

Left control Bullet connectors
Br/B
W
B
O/W
B/W

The stock key switch has an integral fork lock mechanism at the headstock.  The electrical block connector is six position w/5 used if I remember correctly.

From looking at the wire diagrams of the K7 and supersport variants, I believe there are subtle difference between the two harnesses.

What is your goal?  To get it working/running or restore to stock?  It would seem neither of your bar controls are stock K7 items.

What electrical problems are you trying to solve?  If you focus on individual issues, you can parse the problem to specific circuits and restore operation on a piece by piece basis.  I don't think it is wise to look at the whole electrical system as one big single component replacement, unless you are doing a full-on restore.

Cheers,

[TwoTired]

1) The starter doesnt work.  Mainly because of the grafting of a pre-77 starter switch+relay onto the harness.  The relay just chatters.
2) Other electrics are weird as well, some working well, some not so well, some intermittently.

Both these could be low battery voltage related, or a starter that is drawing way too much current in the first case.  If you don't have a multimeter now and expect to do an electrical refurb, you really ought to get one.  They can be had for $10-20.  And it is an invaluable tool.

The relay chatters because the when it energizes, the system voltage drop due to huge current draw, and that release the electromagnet in the solenoid. When it disengages the voltage goes back up and re-energizes the solenoid.  The cycle repeats, making it chatter.

3) The more I stare at this wiring diagram, the more I realize that its a 77 harness, and the connectors for the non-77 controls and iginition grafted on to get them working.  To top it all off, the hackjob isnt even a good one.  Wire colors are wrong, the soldering is #$%*ty, and they used the most generic connectors possible.

Unless you replace all the components with new, the replacement parts are STILL going to have 30 years worth of oxidation on the connectors.  Which you should clean and lube to restore reliability, anyway.  Certainly you will have to correct/redo whatever hack has been made to restore confidence.  That is a given.

4) When I'm 500 miles from home and this homemade electrical taped nightmare of a wiring harness takes a crap, I'm going to wish that I had just sprung for a good factory unit right off the bat.

  Unless new, a replacement harness is not a panacea. 30 years of oxidation just happens. There is still a lot of electrical parts on the bike that needs to plug into your replacement harness.  What of those connections?

At the end of the day, my main concern is neither a complete resto, or just getting it running.  My main concern is reliability, since I plan to tour on this bike.  If it breaks down on me, I want to be able to reference a factory Honda wiring diagram and start troubleshooting.  I really dont want to have to second guess everything because its all kludged together.

Totally understand the reliability issue.  With you there completely.  I'm just trying to save you some money on parts cost. Reliable repairs are certainly possible to make.  I have repaired several harnesses using bits from a harness donor.  It doesn't necessarily have to be the one for your particular bike.  Honda used a repetitive color scheme for most of their bikes of the era.  The block connectors will likely be your biggest issue.  You might be ale to get them from vintageconnections.com.  Even if you get them with a replacement harness, you still need the mated partner for the bar controls.  Unless you buy them too, of course.

Cheers,

[754]

Found the Harness,it has a part # tag on it.
32100-405-0001   (I dont have partsbook for those years)

It appears to be a Honda harness by wire connectors and colors, and appears to have box connectors and bullets. I also located a left side switch housing and wires that look like a 73 except the main connection is box type (did not check to see if it was external wires but must be?)

[TwoTired]

The 405 is a 77-78 CB750K product code.  Looks like is should work.

Cheer,

[TwoTired]

1. Make sure your battery is well charged.  Weak batteries won't start a bike, even if primo condition.

2. To directly bypass the solenoid bridge the two big post with something heavy metalic. Shield eyes from sparks and make sure it's NOT in gear.  If your starter motor spins...

3. Check the solenoid itself by disconnecting the two smaller post terminals and connecting them directly to the battery terminals with some jumper wire.  If the starter motor spins, both starter and solenoid are good.

4. The bike wiring should provide +12v from the starter button when pressed, on Y/R... At one solenoid terminal.  Don't know what bar control you have so there could be something creative up there.

5. The other solenoid terminal gets Battery return (12V minus) from the bike wiring when either the trans is in neutral, or the clutch switch is activated (pulled in).  Its' G/R at the solenoid.


Cheers,

[mick750F]

You should be able to find a wiring diagram in one of the links in the FAQs. The part # for an F2/3 harness is an F2/3 specific #. Not sure what would make it different than the K...

Mike

Where are you finding all the part#'s are they listed somewhere?

   I have a parts fiche for the F2/3...

Mike

[TwoTired]

1) Its not on a battery, I'm using my charger in high amperage mode

Unless your charger is capable of 150 Amps output. I'd say that's your first problem.

5) I have the clutch switch unhooked right now, would that stop the starter from turning?  Even when I directly jump the solenoid?

Not if the neutral light is working.

I seriously doubt your charger has enough poop to turn the starter motor.  What is it's amperage rating?  Can you weld steel with it? 

Cheers,

[TwoTired]

Hmm, well, tomorrow, I'll hook the bike to my car battery with jumper cables.  If that doesnt do it, nothing will.

Yes, that should do it.
Be sure to observe the proper polarity connections.  Less than a quarter second of reverse polarity will destroy your rectifier. And then later, smoke your wire harness.  Then you'll have to replace both for sure.

Also, don't operate the starter motor for more then 30 seconds continuous, without a cool down period.  I can overheat and well, let's not replace that if we don't have to.

Why are you avoiding the proper battery for it?  You can't really operate the bike without one, you know.

The neutral light comes on when the neutral switch closes providing a return path to both the light and a starter solenoid terminal.  Then the starter button can put +12v on the other terminal and the solenoid can engage the starter current.

Cheers,

[754]

Sounds like you may not need a harness.

When they changed the switch they either  cut off the end conector from the 77/78 wiring  and grafted it to the 72 and earlier switch, or vice -versa.. but even if they did cut your harness end off it may be ok.. just check that the connections & splices are sound