Topic: Please help on this CB750K (even though it is a DOHC)

[CBR_Rod]

My father-in-law bought a 79 CB750K,  Pretty clean, all original, 12K on the clock.  We went for a ride the other night and as I let him pull up to me so I could tell him how dim his headlight was, the bike died.  We push started it, and that nearly got him home that night, but a second push start would not immediately start the bike, he had to wait a few minutes I guess.

Anyways, I searched the forum for info on testing the Stator and found that apparently I needed to check voltage on the 3 yellow wires coming off the stator.  Well, we just got through checking them, at the point where they go in to the regulator/rectifier and all 3 would read 14 to 15 volts (checked on the AC side) at idle and would get upwards of 22 to 23 volts at 5 grand.  Near as I can tell that is about half what it should be putting out.

I checked the 3 wires against a frame ground and the black wire ground as well, the above readings were from the frame.  When checking with the black wire I'd get less than a volt like .6 to .8 volt. 

Does this sound like a failed stator? 

One other question, what is the white wire that ran grouped together with the 3 yellows and 1 black wire? 

He charged the battery to get the bike going.  When I checked the batery it was just over 12.0 volts with ignition off.  When it was running, it registered 11.2 volts at idle and 11.5 at about 5,000 RPM.


All help appreciated.

[Jinxracing]

The first thing I would try would be a "known" good battery, and see if you get higher voltage readings while running.

[CBR_Rod]

What would a new battery have to do with the stator output?  Can it draw a stator dow or something?

[Jinxracing]

I just mentioned that as a quick and cheap way to start the troubleshooting process. God only knows where the electrical glitch is, but you'll feel kinda silly if you go to the trouble of replacing the stator only to find out that it was just a worn out battery the whole time. 

[CBR_Rod]

OK, thanks, but from what I've dealt with in the past, a good charging system will sustain the ignition system and lights and keep the motor running, with or without a low battery.

As it stands now, the battery gets charged up (with a battery charger) and the bike starts and runs fine until the electrical system drains the battery by one of two means, either a stator not putting out enough volts to sustain the system and charge the battery or the regulator rectifier is bad.  I'm wanting someone that is in the know with stator testing to verify if I have checked out the stator sufficiently or not and if I have do they concur that the stator is bad or am I missing something. 

As for the Battery condition, it is approx. 6 months old and according to my charger with built in tester, when charged, it indicates the battery is good when load tested.

[82CB650SC]

check the rotor resistance should be about 4 - 6 ohms.  it uses the same rotor as the 650s and they are known to short

[Jinxracing]

OK, thanks, but from what I've dealt with in the past, a good charging system will sustain the ignition system and lights and keep the motor running, with or without a low battery.

I'm not going to pretend to be an electrical guru...far from it...but if you do a forum search you'll find that these early Honda charging systems operate differently than you're used to; a low battery will in fact affect how the bike runs, regardless of the state of the charging system. More informed members than me can fill you in as to why.

[CBR_Rod]

check the rotor resistance should be about 4 - 6 ohms.  it uses the same rotor as the 650s and they are known to short

Could you tell me how I should test the rotor, i.e. wires (yellow and black), pull stator cover and check something in there?

[tsp37]

This was a good trouble-shooting guide for me:

http://www.electrosport.com/electrosport_fault_finding.html

[bryanj]

Brushes and/or rotors go bad way more often than stators and without a decent battery voltage/current into the rotor you will not geta decent stator output, which should be over 20 volts AC between yellow conductors if disconnected

[CBR_Rod]

This was a good trouble-shooting guide for me:

http://www.electrosport.com/electrosport_fault_finding.html

Thank you, this looks to be very helpful.

[CBR_Rod]

Using that intuitive guide in the provided link I finally got around to checking out the in-laws CB750K (79 model).  According to the flow chart, the Rotor is bad.  I got to where it wanted me to check the resitance between the two slips rings (I assumed these were the two places the brushes ride on the rotor?).  The book said there should be between 4 and 6 Ohms resitance.  There was no resistance.  I checked continuity between the wire plug and the brushes and it was good.

Can anyone suggest where to buy a new Rotor for this bike?  As I understand it the Rotor is interchangable between the 79-82 CB650, CB750, CB900, and the CB1100.

Thank you for all the help.

[jmuthaf]

 Yeah, i have an 81 dohc with the same problem. The rotors are usualy the suspect for not charging on these bikes. I had my rotor rewound by a guy on ebay, http://cgi.ebay.com/ebaymotors/Rebuilt-Honda-cb-650-750-900-1000-1100-rotor_W0QQcmdZViewItemQQcategoryZ35594QQihZ008QQitemZ180143258836QQrdZ1QQsspagenameZWDVW he uses better wire, and a better epoxy to seal it.

[jmuthaf]

PS -  getting the rotor off can be a pain, it is a pressed on taper fit. you can use the rear axle, it is the same thread size, thread it in and smack it with a hammer a few times, tighten it, smack it tighten it... and it'll pop off.

[CBR_Rod]

Thanks for the link and recommendation.  I forwarded the link to my father-in-law.

[CBR_Rod]

Well, here is what is happening so far.  Some sneaky fellow jumped at that provided link on Ebay and it was sold by the time my father-in-law found it. 

Anyways, that same seller has prepaid auctions where my father-in-law is sending in his Rotor to be rebuilt.  In the mean time I order him new tubes and tires for his bike and this give us time to change them.