Topic: Charging goes nuts at 4K - 5K RPM... SOLVED!!!

[AndreRA3]

Hey folks,

It's been a while since I've posted, but a quick search didn't show any answers to this problem, so here it goes:

I have a 1974 CB750k frame with a 78k engine (and by the looks of it wire harness is 78) with electrical problems.  Right now the problem is that the bike overcharges (as in 16v +) when it hits 4-5000 RPM, after that it stabilizes until the 8000RPM point.  What could be causing this?

Since I've had the bike I have done quite a bit of work to the electric system trying to get the bike to charge correctly, which it did for almost 2000 miles.

It have:
Checked the wiring harness for shorts, bad contacts (found nothing but isolated some things just in case)
Put in an Electrex Regulator/Rectifier unit
Replaced the alternator with a used one (checked with a multimeter and OK)
Installed a Kuryakin battery gauge that reads off the wire harness (it's how I noticed the overcharging)
I have a Dyna-S ignition and Accel 3 Ohm coils as well.

I have a sneaking suspiciion that since this problem is RPM dependent, it has something to do with the ignition or coils.  Any clues?  Am I missing something? 

BTW, all fuses seem good.  And speaking of fuses, which main fuse does a 78k bike take?  I have a 30 amp one in (P.O. had it there) but the Fuse box calls for a 10 A (74K I suppose?)  Could this be the problem?  Have I asked enough questions? 

André

[kghost]

Sounds like a regulator problem to me.

The charging system doesn't put out alot of power at low rpm. Can't remember the exact RPM were the system hits 14 volts, seem to remember 3000 RPM as about right. 16 volts at 4-5000 rpm is what it would put out if the regulator wasn't working.

Sometimes even new they just die.

[AndreRA3]

I thought about that, but the thing drops back to 13-14 volts at 5500 RPM!!!  I have raised the RPMS slowly, going through the entire range, and it goes insane only between 4-5K RPM, but goes normal (and stays normal) as soon as the RPMS raise or lower...

[kghost]

The new Solid state regulators aain't like the old ones. Sounds like yours has issues.

[eldar]

I had a problem like this on an old car of mine. The voltage would rise as the rpms went up. I blew headlights like nuts. I thought is was an alternator. Swapped it since I had a spare, no change. Changeed the wire harness and BANG that was it.
Point is that it could just be the harness. Otherwise regulator sounds like the deal.

[TwoTired]

Carefully check the black and green wire connections at the regulator all the way back to the battery plus and minus.  Although I am not familiar with your specific regulator, it likely uses these wires to monitor battery voltage.  If there is a fault in these two wire circuits, the unit will be controlling the field coil, and thus alternator output, based on faulty information.  Compare the voltage reading between the Black and green wires at the voltage regulator to the voltage readings at the battery terminals.  Use two meters if you have them.   Otherwise, chart each measurement point over the RPM range and compare the charts.

Be aware that the alternator full output is variable with engine speed.  If you have placed extra loads over stock configuration on the bikes electrical system (3 ohm coils, higher wattage headlight, etc), it will require a higher RPM to generate enough power to overcome the load and begin charging the battery.

The main fuse is sized relative to the current draw expected from the end use devices.  The stock main fuse for the K4 was 15 Amps.  Adding more electrical loads will errode the safety margin for the fuse trip point and cause it to run at a higher temp.  So, if you add load, you should increase the main fuse rating by whatever additional nominal load added.  For example, 3 ohm coils add about 3 amps to the bike electrical load.  So, you would go to an 18 amp fuse to compensate.  Or, whatever the next size rating is available, probably 20A.
I suspect there is a 30A fuse in there because the lower rated one was melting.  Search for the topic "wtf fuse box melted" or similar on this site.  You will probably have to clean the fuse contacts of oxidation.
Good luck!

[oldbiker]

I agree with the opinions that the regulator is suspect, but another point occurs to me. If this problem only occurs at one particular rev range it could be that the sensor wires are resonating at that vibration frequency affecting any connectors and thus giving false information to the regulator. Taping them down to something solid might help to stop this effect.

[alfabeast]

Double check with a good meter like a "Fluke" , I woudn't trust that Kuryakin gauge, it could be picking up noise from somewhere, You've got top notch electrics on your bike, except for that chinese made gauge, it may be shiney but "everything that glisters may not be gold"

[Badboy]

If the regulator is working like it should it would not go to 16v.  Unless you have a short to the [batt+ to field ]. I assume that you have a exciter type set-up.  I think from what you have said,I would get a voltage reading on the field terminal of the regulator. If that is full battery voltage[at all times]  ,two things ,regulator bad or it is getting batt+ [the field wire] somewhere.

[AndreRA3]

Alright, problem solved!

I did a clean up of the hiring harness and (probably the biggie here) recharged the battery.  It was at 12.5 volts...  Now at full charge, the system reads 12 votls up to 3000, +- 13 volts at 3-4K, and 14 volts after that.

I rode the bike all day today, with no problems at all.

Thanks to all who helped,

André

[AndreRA3]

By the way, I'm selling the bike 

I'm going to wait a couple of weeks to make sure all is well before "officially" announcing it in the for sale forum, but I already have signs taped to my bike.  As much as I love this bike, I need to pay for tuition, and August is just around the corner.  This is my 2nd bike right now, and having to struggle to pay for college with an extra bike seems silly, so it has to go.

Thank you to all who helped me with this.  I refuse to sell a lemon, and want(ed) to make sure all was 100% before passing it on to a fellow rider.

If anyone is interested, it is a 74 cb750k frame with a 78k motor.  It has
5 gallon dunstall (replica?) tank
fiberglass 2 up seat (also dunstall)
coated 4-1 headers with performance muffler (kerker, perhaps)
Emgo Pod filters
Dyna-S Ignition
Accel 3 Ohm coils
Electrex USA Reg/Rec
Redone alternator
Redone clutch (less than 3000 miles ago)
Dual front disk conversion (supersport setup)
comstar wheels
Clubman bars with barend mirrors
mini tach and speedo
L.E.D. panel lights, brake light, front blinkers
halogen headlight
NEW rear tire (dunlop 491 Elite II) with less than 300 miles
and a few more goodies

Asking price is US$ 2,200.00 with a strap on tank bag, bubble fairing (no hardware, though), extra parts like supersport airbox with a k&N inside, extra stator coil, etc...
Clear texas title

Just thought I'd write it here, sorry if it doesn't belong.

André

[kghost]

Photos, we demand photos.

Also where in Texas is the bike?

[AndreRA3]

Photos will be provided...  I'll take some tomorrow.

The bike is in Plano, TX, which is just north of Dallas.

The bike will need a paint job to be in top cosmetic shape, but the pics will show that.  I'll try to be as detailed as possible, showing all the good and bad if possible, when I take the shots.

[kghost]

Cool,we await.

[AndreRA3]

Alright, I went ahead and posted it on the For Sale forum with LOTS of pictures.

Here's the link:
http://www.sohc4.us/forums/index.php?topic=2600.0

André