1977 KZ750B Twin. Thought it was Carbs

• Ive had my KZ750Twin for 2 years now. When I first got it, I put in a lithium battery and the proper voltage regulator
• 2 months ago, on a hot day the coil failed because I had not propery secured it. Ive heard loose coils overheat. I changed it
• The other day, while riding, the bike surged like crazy, almost like a bucking horse. Some said it might be the tank vent or a air leak. I checked those out. Its not the tank vent or an air leak
• I put the multimeter on the battery and it charged at 14.77 at 2k rpm (its lithium). However I did notice that, while idling, the voltage would go from 14.77 very quickly to 13v and come back up. Then, after a while of idling it would drop down to 9v for just 1 sec and come back up.
  The bike would for a LONG time without dips in voltage, but the dips did come back slowly.
  Ive been told that the voltage regulator is the issue that caused the surging and bucking and shut off of the bike the other day - not shut down and lose all power, but it would stop running though all the lights were on. Sounds like the voltage regulator.  Do you agree?

Correct, a voltage drop down to 9 Volts do disturb the ignition electronics/timing and coils to generate a strong spark to ignite the A/F mix.
 

Shouldn't the battery act as a damper, smoothing out spikes in the charging voltage?  Said another way, as long as the battery is good, the coils should see a steady voltage even if the R/R output is not consistent.

But if what you say is the case, why then would anyone need a regulator/rectifier?

wrote:

• Ive had my KZ750Twin for 2 years now. When I first got it, I put in a lithium battery and the proper voltage regulator
• 2 months ago, on a hot day the coil failed because I had not propery secured it. Ive heard loose coils overheat. I changed it
• The other day, while riding, the bike surged like crazy, almost like a bucking horse. Some said it might be the tank vent or a air leak. I checked those out. Its not the tank vent or an air leak
• I put the multimeter on the battery and it charged at 14.77 at 2k rpm (its lithium). However I did notice that, while idling, the voltage would go from 14.77 very quickly to 13v and come back up. Then, after a while of idling it would drop down to 9v for just 1 sec and come back up.
  The bike would for a LONG time without dips in voltage, but the dips did come back slowly.
  Ive been told that the voltage regulator is the issue that caused the surging and bucking and shut off of the bike the other day - not shut down and lose all power, but it would stop running though all the lights were on. Sounds like the voltage regulator.  Do you agree?

I would advise caution with this, Is it a three phase stator ( three wires back to the regulator) a drop from 14.7v to 9v is approx 1/3 (one phase) battery voltage can't drop and immediately return. it sounds like one of the phases in the stator is shorting to earth intermittently and pulling the system voltage down WARNING : is there a fuse between the regulator and the battery (30amp advised - not fitted standard but recommended with Lithium battery) although the battery probably has it's own internal protection circuit the fuse is a worthwhile addition. A short of this nature (if that is what it is) can cause what is reffered to as a "thermal runaway" in a lithium battery which can lead to a battery fire (it is almost impossible to extinguish these and they have to burn themselves out normally)

the rectifier has two yellow wires, one black and one red.
 

wrote:

the rectifier has two yellow wires, one black and one red.

 

Ok so it is two phase, voltage can only bounce back like this due to an intermittent short pulling it down (if the regulator output was lower than battery voltage your reading would be from the battery and would simply remain the same with or without the engine running. Will look at a diagram in the morning (it’s 1:00am in the UK)

wrote:

wrote:

the rectifier has two yellow wires, one black and one red.


 

Ok so it is two phase, voltage can only bounce back like this due to an intermittent short pulling it down (if the regulator output was lower than battery voltage your reading would be from the battery and would simply remain the same with or without the engine running. Will look at a diagram in the morning (it’s 1:00am in the UK)

Two wires is actually one phase.  Don't ask me how, that's just how it is.  Lou corrected me on this detail when I also said two phase, so it must be true!

wrote:

But if what you say is the case, why then would anyone need a regulator/rectifier?

No, it doesn't work this way.  

If the battery is putting out 12.5 V, and the R/R is putting out 9 V, the bike will see 12.5 V...until the battery power drops.  It's only when the charging system voltage is above the battery, that the bike will see that voltage.  This often happens when the regulator side of the R/R fails; voltage spikes up to 17 V and starts frying stuff.  

Then does my reading mean the Reg/Rec is OK? The highest charging when up by was 14.77. at 2K RPM it was like 14.30. And would voltage make the bike buck like a horse? I mean literally, the other day it was sputtering and bucking and it almost...well almost, felt like I would be thrown off. Jerking forward really hard. Never had that happen on ANY bike before.

wrote:

wrote:

the rectifier has two yellow wires, one black and one red.



 

Ok so it is two phase, voltage can only bounce back like this due to an intermittent short pulling it down (if the regulator output was lower than battery voltage your reading would be from the battery and would simply remain the same with or without the engine running. Will look at a diagram in the morning (it’s 1:00am in the UK)

 

I am more interested in knowing whether an electrical problem would make the bike buck like a horse and jerk forward while riding. I have never had that happen on any bike before

In terms of the readings, I will have to go back and check again to see what the readings are. I was in a rush and may not have done it properly. and was also distracted by family while I was checking. 

The answer is NO, a bad charging system won't affect how the bike runs until the battery discharges.  And when that happens, it's common to drop a cylinder, the headlamp goes really dim, turn signals won't flash, the electric starter won't engage, stuff like that.