Topic: 1977 Honda CB750K not charging

[SigEpRider]

1977 Honda CB750K
Stock airbox
Delkevic exhaust
Stock points
New Wire Harness from Vintage CB750 (original was hacked up)
Solid state regulator/rectifier (installed by previous owner)

My battery is not charging. Here are the steps I have taken so far:
A) charged AGM battery. Verified 12.8 VDC with multimeter. Even had battery tested (tested good)
B) installed battery, started motorcycle and checked battery voltage at different rpm’s - volts did not rise at all - continuously read 12.0 VDC. So this confirms that the battery is not charging.
C) checked field coil resistance - 7.0 ohms
D) checked stator coil resistance - .4 ohms (yellow to yellow wires)
E) checked rectifier portion of R/R - diode test passed
F) ran motorcycle and measured VAC from each yellow wire from the stator - no volts or voltage varies.

With the tests I’m an convinced the regulator section of the solid state regulator rectifier is fried or non functional. I haven’t had a chance to connect a jumper wire direct to the field coil.

Is it possible to have a cheap R/R that is fried?
Should I replace R/R?
Did I miss anything in the test?

I just want to get the expert advice I can find here before I spend money on an electro sport regulator rectifier.

[SigEpRider]

Additionally,

Can a weak charging system cause weak spark?

I’m also dealing with the bike hesitating when I blip the throttle off idle. Sounds like it is sucking a lot of air. I think it might be weak spark, but I want to fix the charging issue first.

[scottly]

Check the voltage at the black regulator wire, then the white regulator wire with the key on, motor not running. Also, you might want to remove the headlight and taillight fuses while you are trouble-shooting to reduce the drain on the battery.

[SigEpRider]

Will do.

What voltage am I looking for?

[HondaMan]

Have you perchance looked at the 3 bullet connectors in the Yellow (under the gunk...) wires from the alternator to the bike's (8-circuit) white plug? These are frequently burned. To get to them you must remove the sprocket cover on the engine, and they are running alongside the inside top edge of the tranny cover. There are also the 2 wires to/from the regulator there: those plugs generally are OK, but can be gunked-up from chain lube and grit.

And, a special note: the K7/8 sprocket cover can be tricky to reinstall. Hand-thread the lower screw in first, then the upper one, and snug down in that order. I frequently run a 1/4" drill down thru both of those holes before reinstalling this cover, because you can actually break it if it is misaligned while tightening on the K7/8 engines. It appears that the jig that held this cover for drilling those holes during the K7/8 era was at a slight angle for the upper screw, which causes this trouble.

[SigEpRider]

Removed fuses. Here are my readings

Black wire 12.0 VDC
White wire 7 mVDC (don’t know why my multimeter reads m V)

[scottly]

You should have 12V going into the regulator on the black wire, and a little bit lower coming out on the white wire to the field coil. If you have 12V in, but 0V out, the reg is bad. You can also do a quick-and-dirty test of the reg by checking for magnetism on the field coil mounting screws.

[SigEpRider]

Have you perchance looked at the 3 bullet connectors in the Yellow (under the gunk...) wires from the alternator to the bike's (8-circuit) white plug? These are frequently burned. To get to them you must remove the sprocket cover on the engine, and they are running alongside the inside top edge of the tranny cover. There are also the 2 wires to/from the regulator there: those plugs generally are OK, but can be gunked-up from chain lube and grit.

And, a special note: the K7/8 sprocket cover can be tricky to reinstall. Hand-thread the lower screw in first, then the upper one, and snug down in that order. I frequently run a 1/4" drill down thru both of those holes before reinstalling this cover, because you can actually break it if it is misaligned while tightening on the K7/8 engines. It appears that the jig that held this cover for drilling those holes during the K7/8 era was at a slight angle for the upper screw, which causes this trouble.

No I haven’t. But I’ve been testing the stator and field coil at the block connector that comes out from there. The tests passed, so I assumed that it was an issue in that area. However I will look at them tomorrow.

Thanks for the suggestions on the sprocket cover. You’re right. It’s a pain.

[scottly]

Removed fuses. Here are my readings

Black wire 12.0 VDC
White wire 7 mVDC (don’t know why my multimeter reads m V)

Your auto-ranging meter is reading millivolts, which is damn near zero.

[SigEpRider]

You should have 12V going into the regulator on the black wire, and a little bit lower coming out on the white wire to the field coil. If you have 12V in, but 0V out, the reg is bad. You can also do a quick-and-dirty test of the reg by checking for magnetism on the field coil mounting screws.

Interesting. Is that with the motor running?

[scottly]

No, the magnetism test is just key on, not running. If no magnetism, no charging.

[SigEpRider]

No, the magnetism test is just key on, not running. If no magnetism, no charging.

Thanks for the suggestion. I’ll check that tomorrow also, but I think we can verify that it’s the regulator section. Must have been a super cheap R/R the PO installed. Any suggestions for a new one? Brand/vendor

[scottly]

Sorry, I can't help there; I'm using a modified Ford regulator with Radio Shack rectifiers in a custom wiring harness.

[SigEpRider]

I, sure that’s a post somewhere lol.

Thanks again for the help

[70CB750]

I chased down charging problem on 78SS engine.  It was a tricky one.  She would charge cold but stop charging when she warmed up.   

I eventually went Edward Morris* and replaced the stator with NOS one.

 I also bought new aftermarket sub-harness that goes from the stator to the rectifier/regulator.  Compared to the old piece it was an eye opener, I finally understood how is the harness supposed to fit in the cover, pretty clever actually.   

* Edward Morris build CB750 from NOS parts, quite impressive. 

[newday777]

I chased down charging problem on 78SS engine.  It was a tricky one.  She would charge cold but stop charging when she warmed up.   

I eventually went Edward Morris* and replaced the stator with NOS one.

 I also bought new aftermarket sub-harness that goes from the stator to the rectifier/regulator.  Compared to the old piece it was an eye opener, I finally understood how is the harness supposed to fit in the cover, pretty clever actually.   

* Edward Morris build CB750 from NOS parts, quite impressive. 

Who is this Edward Morris you speak of??

[70CB750]

http://forums.sohc4.net/index.php?action=profile;u=36148

His bike:

http://forums.sohc4.net/index.php/topic,126735.0.html

[SigEpRider]

Just a quick update:

Got a new electro sport regulator rectifier.
Plugged it in.
Batter ready 12.6 VDC
Started motorcycle and let it warm up (I’m having carb issues also)
idle: 12.3 VDC
2000: 12.9 VDC
3000: 13.6 VDC
4000: 14.9 VDC
voltage is .4 - .5 VDC higher than what the manual calls for.
For now, I’m calling this a win. Voltage is now increasing as the bike revs.
Also, I don’t know how reliable my Tach is since it bounces a lot last time I took it for a ride.

Thoughts?
Am I overcharging?

[bryanj]

Is that win lights on? If so fine for a lead acid battery

[SigEpRider]

Yup. Lights off. I forgot to put the fuses back in after checking the voltage regulator.

I’ll check again with lights on.

I’m using an AGM battery also.

[PeWe]

AGM battery need higher voltage than std wet.

Google AGM battery charge.

"AGM batteries are designed to require 14.6v – 14.8v when performing a full charge from a state of discharge, whereas standard lead acid batteries need 13.8v – 14.4v, hence this is what most onboard chargers are designed to deliver – not the 14.6v – 14.8v that AGM requires."

So with headlight always on your bike will be in that area, right?
Check with fully charged battery.

I give my AGM battery a higher charge with my charger now and then in AGM mode  (snowflake on some chargers) even direct after a long ride.
I'll see how long it will last.

[HondaMan]

With an AGM battery aboard, the voltage readings will often be slightly higher than those you find in the old manuals. Yours look OK.