1980 CB750F - Starting Issue Advice needed.

[motogrady]

Ha, there ya go.

[eric moon]

Seeing the state of the bike when you got it and your description of the fault, I would suggest that there is a bad connection somewhere. If a contact or connection has a high resistance due to corrosion a high current device, such as the starter motor, will not run.
You mention that the fuse box on the handlebars gets hot. This should not happen with good contacting fuses (high resistance =high current=heat).
I suggest checking ALL connections on the bike. There may be other hidden problems waiting to happen. You dont want that when you are riding in the middle of nowhere!
Eric

[epfromnyc]

Okay all... here's what i found last night. Battery is delivering 12.57 volts to the starter solenoid magnetic switch. So voltage heading directly to the starter solenoid is good. All lights (front, rear, tail lamp and headlamp) are BRIGHT. Turn signals all blink and flash nicely. Headlamp low and high beam switch works nicely. I checked ground on the bike and it seems good.  I cleaned the GREEN ground wire (screwed to ignition coils mounts to the frame) and that connection is good. Ground wires all test successfully when touched with continuity checker and to the frame of the bike.

Just to make sure of ground to the frame... I attached an addition cable to the negative side of the battery and clamped it to the body (making a solid connection). So now moving on to the start run button.

1. I press the start run button and Headlamp dims (as it should) and the returns to normal brightness.
*** Voltmeter shows no current from the starter solenoid connection leading to the Starter hot lug.
There is good current going from the battery to Battery side of the starter solenoid. The first connection point of the battery should be good for current (unless wire is bad).  I checked the starter solenoid for contuinity at the moment that I press the start run button. My thinking is that I should get continuity between both leads on the starter solenoid simply because its a switch that engages and creates a connection delivering power to the actual starter.  I did NOT get any continuity from the B and M connections while holding down the start run button. I did not get any continuity from the B and M connections while checking the starter solenoid without any power to it (which I expected).

Notes: On the starter solenoid. This one is a used replacement purchased on Ebay and listed as good working order. The 30amp metal connector fuse is intact but that could have been installed for the sale.
- The orginal Starter solenoid was suspect to me since it had blown the 30amp metal connecter fuse. I had also tried a cheaper New starter solenoid which has a modern yellow plastic 30amp auto fuse mounted nicely on top but as soon as I connected it the wires on the bike started to COOK. I immediately disconnected it and it seems that the 4 prong connections might be reversed? The wires do not cook when the 4 prong adapter cable is rotated 180 degrees and connected (making the cable appear backwards based on the position of the plastic lock latch facing towards inside of the bike).


Okay so now I have 3 starter solenoids to play with... all 3 are questionable. I will need to really test them.
Obviously the brand new after market one... should work but I am not feeling good about connecting it after the experience with 4 prong plug.

- Starter Notes: The starter works and spins the motor. I jumped th starter ( by passed the starter solenoid) and it operates. Also very nice compression sounds.


Question are there other components that work to engage the starter solenoid... I know the ignition key is good (brand new), The start run button and assembly is good, check inside and connections are clean. Neutral light indicators works when bike is in neutral. Oil light comes on at the same time when key is turned to the on position.



Thanks all!

[LukeM]

You _can_ disassemble the solenoid and check the low current and high current connections.  I remember mine having a bit of corrosion on the high side, cleaned them, and it worked better.  Afterward, I realized the starter brushes were toast.  But it was good to check the solenoid.

Good to hear you don't have a seized motor and what sounds like good compression. Also good to hear the starter will spin the motor when you bypass the solenoid.  Looks to me like it's still having problems on the low current side (starter switch, solenoid, run/start switch maybe).  Grounds are verified good, battery is good, starter is good.  Process of elimination.  I'd keep looking at wiring: maybe some little critter nibbled on a bit of insulation, and something's touching ground that shouldn't be.  It IS a barn find, right?

Good luck.  You're getting closer.
Luke M

[epfromnyc]

Yep a barn yard find... time eater for now. We've all been there.  I have every wire exposed on the bike as I needed to know if any one was crimped, broken or exposed. Looks like enough wire to tight rope walk all the way back to japan! Anyway, process of elimination. Once I get pass the starter issue... on to checking charging system (while it's running). Carbs have already been re-done.

[motogrady]

check 'em on the bench, meter set to continuity, hook those to the lugs that
supposed to close and carry the heavy juice to the starter.   the big lugs.
then take 2 pieces of wire, an old ext cord split in 2,
 and jump right off the battery to the little lugs.

meter should show continuity and you should hear or feel something kinda click..

if not, check windings in solenoid, might be an open winding...
or maybe a siezed or jammed set of contacts in the soleniod itself.
if that thing is jammed, or the windings half shorted out,
it would pull a lot of juice if you held the button down for any length of time, getting things hot.

you are getting what i would call control voltage to the solenoid when you hit the switch right?

[epfromnyc]

Motogrady: Yeah I will bench em and test. I did a quick test of the solenoid while on the bike and NO current passed through the lug connected to the starter. But the wire does have continuity and seems good. Didnt hear  a click. Checked on the voltmeter and Zero voltage from the M side lug to the starter.
I only press the Start Run button for about 4 to 5 seconds. Dont like holding them in for too long. I look over at the voltmeter while I press the start button and nothing jumps on solenoid or no sound.  So I would say that I am not getting control voltage through the solenoid.

The start run button and assembly is a used replacement and appear very clean inside and out. The orginal assembly reacted the same way but didnt look as good. I believe they are both functional.

[shinyribs]

Does the dohc bikes have a starter safety diode like the sohc bikes?  

[epfromnyc]

Wow that is a great question. I'm not sure. But I did wonder about while trying to figure out this bike.

---

From: "shinyribs [via Honda CB750'S]" <[hidden email]>

Date: Thu, 6 Sep 2012 14:43:37 -0700 (PDT)

To: epfromnyc<[hidden email]>

Subject: Re: 1980 CB750F - Starting Issue Advice needed.

Does the dohc bikes have a starter safety diode like the sohc bikes?  

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/1980-CB750F-Starting-Issue-Advice-needed-tp4028643p4028784.html

To unsubscribe from 1980 CB750F - Starting Issue Advice needed., click here.
NAML

[epfromnyc]

Okay All: Here are the two solenoids that were part of my starter problems.  The solenoid on the right was the original solenoid. You will notice that there should be 2 copper wire connectors ( very thin wires) that should extend into the metal body. One of these thin wire was cut very short and probably was not making it into the body. So it was doing nothing.  The other solenoid was purchased used from ebay. Looks very nice on the outside, however the metal cylinder is completed corroded and therefore seized. NO WAY That Part could move freely.

Using both these solenoids in their current state yeilded no Power to the starter motor.

SO the plan is to use the good parts from both to make 1 working solenoid.

[epfromnyc]

Here is a pic of the CUT thin wire lead... which normally should feed nicely into a a 2 holed receiver in the metal body of the solenoid.

[epfromnyc]

Here is the corroded closeup.. on what should have been a used working replacement solenoid.
Can you say "HAPPY SHOPPER"?

[LukeM]

Since ribbsy asked about a starter diode (I don't believe these are used on the DOHC bikes), I saw something else:  Electrical stuff from cb750supply

A new Starter Solenoid runs about $26.  Don't know about shipping or lead time, but maybe it's worth a look.  Beats buying used stuff on the EBays.

Luke M

[shinyribs]

If you have enough room to mount it ,you can get a Ford solenoid for about $8 at any auto parts store . It works the same.A solenoid is a solenoid. I bet you could pick up a lawn mower solenoid at Wally World and it would fit.They're pretty small. Last one I got for a Murray mower was around $12,IIRC


Thanks for the diode info Luke.

[epfromnyc]

Hey.. Thanks for the great alternatives. Didn't think about the other solenoid options. You are right.
Yeah, I did try a brand new after market one.. but cooked the wires leading to the spark units... it didn't like the 4 connection cable adapter. I inserted just like on the OEM solenoid but the after market is wired differently. So I bailed on it.  Didn't want to take another chance on causing more headaches.

I did complete the rebuild solenoid used parts from both mentioned before... looks nice and should work.

[epfromnyc]

Hey The DOHC cb750s do have a diode. Just realized which wires cooked and what they connect to. I will send a pic of that diode. Looks very much like on SOHC.

[epfromnyc]

Here is a pic of the diode. The wires to the diode were cooked when and after market solenoid was installed on the bike. The diode was tested with ohms meter... and is functioning.

[epfromnyc]

Heres my baby

[epfromnyc]

Work in progress!

[shinyribs]

Look at all the pretty colors! Like a rainbow!   I'm sure that not what's been going thru your mind lately:)