Topic: 77 CB750 Problems

[Parry]

Hello, I have been researching this forum for months as i have been building my bike. I have come across a problem and couldn't find a solution on the forums.

My bike seems for have a odd electrical issue where the battery voltage fluctuates rapidly. I have included a video below to illustrate the problem. The battery and regulator/rectifier are brand new. While the bike is off or ignition ON position, the battery is stable. while the engine is running the battery voltage fluctuates between 0.67v-12.6v rapidly. also while revving, the battery doesn't go above 12.6v so don't seem to be charging.

Tests i have conducted, I initially thought it would be a Stator problem, so i got my multimeter and my results (minus the tool resistance), the 3 yellow wires are getting 0.5ohm each (from the connector spot), the Field Coil is getting 7.2ohm I have also tested grounding each wire and everything is fine. so i believe the stator is not the issue.

the second test i did was on the new regulator/ rectifier. I preformed a positive and negative bias test using the diode setting on the multimeter. it seems to pass the test. and i didn't find any problems.

I do not know any other test i can preform to test the issue. Any help will be greatly appreciated.
Thank you!

[rotortiller]

Might be the ignition switch. Sometimes the contacts are sensitive to vibration. I would look at a loose or bad main power connection or fuse holder. If you sample voltage at the battery terminals maybe the meter leads are at fault or the battery (unlikely though).

[maxheadflow]

A good battery cannot go from 12.6 volts to 0.67 volts without there being smoke or heat somewhere.  It's more than likely you have a connection issue.

It does sound luke the charging system is not working. Measure the voltage across the field coil.

[Parry]

A good battery cannot go from 12.6 volts to 0.67 volts without there being smoke or heat somewhere.  It's more than likely you have a connection issue.

It does sound luke the charging system is not working. Measure the voltage across the field coil.

i did a test on the field coil, they showed 7.2ohm. when you say measure the voltage across the field coil. how exactly do you mean?

EDIT: i put the bike in the ON iginition position, and the white and green wires leading he the field coil read about 10.8V

[maxheadflow]

With the ignition on and the bike not running, IIRC, it should be at battery voltage.  If your battery voltage is only 10.8 with the ignition on it's pretty weak.  If above 12V at battery, start looking for voltage drops.

Start the bike up and look at what the feild voltage does. 

My bet is that you will need to go through all the electrical connections and clean them up.

[754]

 Are you in Vaughn or Kleinburg ? Welcome aboard..

[Parry]

With the ignition on and the bike not running, IIRC, it should be at battery voltage.  If your battery voltage is only 10.8 with the ignition on it's pretty weak.  If above 12V at battery, start looking for voltage drops.

Start the bike up and look at what the feild voltage does. 

My bet is that you will need to go through all the electrical connections and clean them up.

My battery is starting to die with the constant starting but, it was 12.45v and the field coils were at 10.08v, when i started the bike, it dropped to below 10v and started fluctuating again. I also checked one of the black headlight power wires, it was getting about 10.6ish V at ON position. im going to charge the battery now.

[Parry]

Are you in Vaughn or Kleinburg ? Welcome aboard..

I'm in Kleinburg, Thanks!

[Parry]

Some additional tests i have done this morning, Battery is about 90% charged showing surface charge of 13.5v, turned the bike in ON position, i placed a wrench near the stator and felt a magnetic pull, so i believe the field coil is working correctly. i changed the main fuse, i noticed the fuse was a 14A fuse and i put  a 14 back in, the fuse box says it should be 15A, not sure if this makes a difference or is my problem. anyways, i started the bike, and have the same problem, low fluctuations in voltage from battery.

any ideas i can try?

EDIT:

I noticed a small leak on the new battery. it may be damaged. could be the reason it is jumping from 13v to less than 1, I'm going to attempt charging my old dead battery, and test the bike if i get voltage fluctuation.

also question, say my stator wasn't working, and my battery had full charge, the bike wouldn't cause the battery voltage to fluctuate rapidly  right. the voltage would steadily over time just go down till it died right?

[maxheadflow]

When the voltage goes down to 1, does the motor quit?

[Parry]

When the voltage goes down to 1, does the motor quit?

no the engine never dies, but my tail light flashs on and off and my speedo flickers and so does the headlight

in the video you can see the voltage jump to as low as 0.67V and the bikes running

[maxheadflow]

Well you've got a loose connection somewhere.   Where was the voltmeter connected?  Motor won't run on 0.67 volts to the coils. Taillight flashing could be a bad ground.  Since it looks like you went through and painted a bunch of stuff, I'd look for poor grounds. One ground that is often missed is one to the forks / handlebars.  Bike is old. what year wiring harness?  Does it have a headlight switch?

[Parry]

I have replaced the battery just to be sure it wasn't a faulty battery. the battery started with 12.8v surface charge, when i started the engine and the revs started to pick up, the battery voltage went up to 14.79V, this tells me that the charging system is working properly. but i still getting random Volt Drops to between 0.68 and 14.79V. the video below illustrates this.

Well you've got a loose connection somewhere.   Where was the voltmeter connected?  Motor won't run on 0.67 volts to the coils. Taillight flashing could be a bad ground.  Since it looks like you went through and painted a bunch of stuff, I'd look for poor grounds. One ground that is often missed is one to the forks / handlebars.  Bike is old. what year wiring harness?  Does it have a headlight switch?

The multimeter is connected directly to the battery in this video, all the grounds go to the lights go to the green wire which i think grounds to the screw near the coils. The frame of this bike was powder coated. it may be possible that harness ground may have a bad connection? ill check that next.

[maxheadflow]

Do you have another set of test leads?  Try pulling on all the test lead connections.   You're bike is charging.  Like I said, battery voltage can't go that low then the bike running.

[Parry]

I checked the main harness ground, sanded the frame as well. its not the problem.

Do you have another set of test leads?  Try pulling on all the test lead connections.   You're bike is charging.  Like I said, battery voltage can't go that low then the bike running.

i will go buy another multimeter and test this next.

[Parry]

so i bought a new multimeter and did the battery test. the voltage did not fluctuate anymore, i only tested for about 2 mins because it got late. but ill test again tomorrow to make sure everything is ok. I have included a video of the new multimeter test. it also shows good charging.

[maxheadflow]

I bet the tail light flicker is a bad ground.. It's real hard to troubleshoot when your test equipment is on the blink..

[Parry]

ok, so the only problem i have left is me speedo, everything works while its in the on position, while when i start the bike, the numbers start to flicker and go out. The video better illustrates this.

[maxheadflow]

Again,  Make sure that the power and grounds are good to the speedo.  A common problem is not grounding the trees to the frame.  I don't know anything about that particular speedo. Could one of the inputs be seeing too much voltage.  Is there a Tach input?   Speedo may be simply bad. 

[Parry]

Again,  Make sure that the power and grounds are good to the speedo.  A common problem is not grounding the trees to the frame.  I don't know anything about that particular speedo. Could one of the inputs be seeing too much voltage.  Is there a Tach input?   Speedo may be simply bad.

I'm going to attach the speedo positive and negative directly to the battery and test it tomorrow.
I was wondering . Say my battery was getting 13v ti the battery while running. How much V should be at the headlight.

[maxheadflow]

With 13 volts at the battery, it would be nice to see the same number at the head light.  I probably wouldn't worry if the drop was 1/2 volt but it might be a little more depending on how good the connections are.

[Parry]

still haven't figured out the speedo yet, I'm working with the seller to see if its a speedo issue.

I have started to tune my carbs, Its not 100% there yet, I have included a video for reference. when i rev, the revs hand high for a bit (15 second mark). I tightened the rubber boot to make sure its not leaking air in. when I'm at speed, it seems to hesitate after say 6000 RPM (1:10 mark). 

Carb setting,
Stock pilot
120 Main
2 turns out air fuel
needle in middle position
altitude about 500 meter

I have bench synced the carbs. The Accelerator Pump is working and all 4 shoot gas. carbs were cleaned thoroughly. I haven't looked at the plugs yet (ill report back on that). judging from the video. do you think the needle position is to rich?

[Parry]

I just came back to the bike after a week and i installed Exhaust wrap. but. when i started my bike. It made a loud slapping or knocking noise? listen to the video for reference. what is this notice? do i need to adjust the cam chain ?

[RAFster122s]

Sounds like an exhaust leak at the exhaust spigots...check the crush “washer” gasket is sealing if you have them.  The exhaust gasket os usually a copper ring that is a hollow tubular section and it crushes to conform with the two surfaces. See if your exhaust is loose or just replace all four...
A rubber hose held to your ear with the other end probing around the motor can help pinpoint the general area of the noise more precisely...