"Balanced" system.  Split AC/DC system.  Battery is only half the rudimentary "regulation".

You need to have the correct wattage bulbs to balance against the AC stator output (headlight) on the early models. 

Best thing is to install a factory Yamaha AC voltage regulator in the headlight circuit.  There is a service bulletin on how to do it. 

Or buy a 3-wire regulator from DEET.

I say original Yamaha because many of the "look alikes" are total crap.  I've found good used Yamaha regulators on ebay for less than $20.  Deet regulator is not much more than that if I remember correctly. 

I'm running a 6v 25 watts for a headlight bulb that should be correct for low beam. Not switching to high beam as the test bulb is a single filament, but that shouldn't matter unless I flip the switch. And a 6v 3 watts for the dash, mostly original bulbs I've collected over years. Yeah, I read that about the service bulletin, and I've ordered one just because I figured it's good to protect that original headlight bulb. But I was just trying to get it to work right without that. It is what it is. I also bought a new rectifier just because. And the battery is yusa original manufacturer original part number; I'm actually surprised they still make it. The help is much appreciated!!

You have (several) 6v 25 watt test bulbs?  That's interesting.

BTW, high and low beam should be the same wattage?.  Filament position and resulting "focus" is what makes the beam pattern shift, not wattage. 

Precise filament location within the reflector/lens is why I've never seen someone "hack" another bulb into a stock reflector and get a usable light pattern.  It might be bright but that's all. 

No, I've put together a test unit using an 1154 scoket and 1154 bulb. I have the original working headlight bulb and don't want to blow it! So, I made a test unit. I know the high beam is 25 watts, but I'm using a single filament bulb to test because I just happened to have a few of these 1154 bulbs, so I'm just not switching to high beam. The original bulbs I have are a bunch of the little 3-watt dash or speedometer bulbs. I actually have all kinds of weird bulbs because of the old cars I work on like current we are working on a 28 Lincoln Model L

Unless you have at least a 25 watt load on the AC headlight circuit, or a good AC regulator, the voltage will spike too high when engine is revved.   With no bulbs connected you'll probably hit 30 or more volts. 

1154 should be a dual element bulb?  I doubt an 1154 "brake" element is more than 15 watts or so?  The tail element is maybe 5 watts...  so even with both elements lit at the same time still under 25.

Magneto has fixed magnets so is fixed output.  Before semiconductors were cheap and available all the manufacturers could do is match the bulb load against the output of the magneto to keep the voltage in check.  And that's also why these systems are 6v...  easier to regulate voltage with load. 

Voltage is somewhat like pressure in a water hose.  Stator is like a water pump.  Bulb load is like a calibrated "leak" in the hose.  Not enough of a leak (bulb wattage) and pressure (voltage) gets too high.  So unless you have a 25 watt load on your headlight wire, or a regulator off a later model properly wired in, voltage will climb too high and "pow" goes the bulb. 

I think the single filament bulb is a 1158...  about 16-17 watts...  shy of the 25 watts needed. 

Bulb says 1154L and says it's a 6v 25 watt. So that's what I'm going off of. The only reason I'm doing this is because it came close to blowing the original bulb. And you can't get them. So, the issue was there with the original light load. The funny thing is it's only the headlight circuit doing it.

Not sure what you have.  Every 1154 or 1154L or 1154LL bulb I find on the Internet or in old paper catalogs I have are 6v dual filament tail/brake not rated anywhere near 25 watts.  Picture would be nice to see. 

Not funny at all that AC headlight circuit is the problem.

It's a split system but the AC and DC parts are "interrelated".  Battery is a fairly good "regulator" on the DC side...  keeps voltage pretty much in check even if AC side is not connected and key is on "night" position...  until you boil all the water out of the battery... 

AC side has nothing to regulate it so if you lose the battery on DC side the extra power available... doesn't take much...  blows the AC bulbs.

One other thought is you mentioned old bulbs...  Old bulbs were generally not rated in "watts" but "candlepower".  Even in the 1960's and maybe 70's.

Possibly the bulb is 25 CP? 

I'm not sure about that. But the good news is I just figured the problem out. It was a bad connection under the gastank. I wiggled the wires under the tank, and the lights suddenly went normal. Wiggle again, and you can repeat the issue. There was corrosion in the connector. Cleaned everything and it's working good now. Still going to put the regulator in just in case. I really appreciate the help! Ypu sa8d something about connections and I was like I'm going to go prod and mess with all the connections really quick. And there it was.