do you think this would fit;

&_ul=GB&mkevt=1&mkcid=28&google_free_listing_action=view_item

found this part number on fleabay 463-82590-21 in singapore

That's for a 1974 model.

The only way to be sure is to buy, and then look in, a parts list.

Wiring oxidized causes HIGH resistance which results in LOW voltage.

Sounds like AC voltage is being measured without load.  Not uncommon to get 100v without bulb load.  System is "balanced" so that with the correct load, the voltage is within range.

The fact that you are seeing that voltage also means you do not have a regulator or you have one of the regulators that look nice but don't work correctly. 

I'm late to the party but is the rectifier connected right through. Is rectifier red live with dc. Is rectifier white live with ac when motor running. Is all DC items working with motor stopped ie flashers neutral stop light horn. Is fuse blown, that'll cause it all. Does white go via the switch so is switch internals corroded.  Have a 12v 35/35 halogen in my magneto LS3 100 & is safe at full song with all other bulbs 6 volt & still charges good. All a wild guess--but!!. I wrote a more long winded story & lost it all. Think this covers it. Man I hate this fone lol.

thank you all for your help I will try everything mentioned - i really appreciate it

Spinning magnets around the stator coils is what makes electricity.  The Yamaha magneto headlight and battery charge output is technically a "Permanent magnet alternator" aka PMA.

Since the magnetic field is constant, the power output is constant.  So say you're riding up a constant slope hill at full throttle and you have just enough power to maintain a perfect 6000 rpm in top gear under that load.  If you would back off on the throttle (reduce power), RPM would drop.  If you kept the throttle wide open and the hill became less steep (reduced load) RPM would increase. 

Now what would happen if you were cruising at 6000 rpm up that hill at full throttle and pulled in the clutch to remove the load but kept full power with the throttle pegged open?  The RPM would go off the scale!

The permanent magnets in the magneto are much like riding with a fixed throttle setting in the example I just gave.  What the engineers did was design the magneto to put out just enough power to keep the voltage around 6 volts under the electrical load.  So if you test voltage with the headlight bulb (load) removed, it's much like pulling the clutch on that hill with the throttle pegged.  Voltage is going to spike off the scale (just like the RPM did in my hill riding example when the load was removed by pulling the clutch).

What we don't know is what your problem is?  Is someone just measuring no load voltage at the headlight?  Or are bulbs blowing or something? 

If bulbs are blowing, you need to make sure all connections are good in the circuit...  don't overlook the headlight beam switch contacts especially if aftermarket**.  If you lose connection to the load (which includes the battery, so make sure you have a good one) the voltage will spike intermittently and can blow the bulbs. 

**Yamaha original headlight beam switch has no "dead spot" when changing beams...  as you transition between low and high beam, BOTH beams are on momentarily during the transition to prevent a momentary "no load" voltage spike during the beam change. 

 

Great write up MarkT.   

hi, i have tried everything but now found the actual copper wire will not give a consistent voltage.
thank you all for your help

found this in USA p/no 463-82590-22