Working on a 1971 ht1. I installed a 6 volt regulator and spliced it into the yellow wire feeding the illumination circuit. I am restoring the bike. I am getting all the wiring straightened out. I ran a jumper wire from my 6 volt battery to the yellow plug where it would hook to the harness from the engine. The engine is not in the bike yet. If I am right the engine will put out ac current not dc. If I connect the voltage regulator it causes a short, is this because of how I have the jumper wire with dc power instead of engine ac power? Or do I have a bad voltage regulator?

Wow, don't hook the battery anywhere near the yellow. Regulator & yellow are all AC power & charging exits the rectifier [little black two wire thing] so battery is safe past that but not before it. Regulator just limits ac volts to save your lights in the eventuality of the battery disconnected & blown fuse or dry of water & save the battery getting boiled dry if ya sitting on 60mph all day long. Miss my HT1.
ps, & if you hook the battery to the yellow you'll likely see smoke coming out of the magneto [lighting coil] so you'll know where the power went. Look forward to hearing how it goes in the end.

Single wire 6v regulator should be "spliced" into the blue wire - link to the Yamaha Service Bulletin stored in the Tech Library is attached.

yamaha-enduros.com/index.php/service-bul...e-voltage-regulators

That's true above--as doesn't get a hard time when lights are off. Now i might be wrong but if you want the charging side regulated to stop overcharging the little battery it need to be yellow related & that way it does two jobs & 'think' yellow arrives at blue in the end through the switch. A good quality ones like the alloy finned IT yamaha regulators can put up with being live all the time --light off & all. [Put me right someone? please]. I used to hook mine to the tail light [blue in oceania models] so it gets everywhere the blue goes, but just had a simple lighting no battery bike. Don't let me steer you down a side street.

Was missing my HT1 & now i've bought a TF100 [mudbug] for $50 & just looked at our 'trademe' to see how much i lost out by on a TF125 in a sad state along with what looks to be an XL185 rolling frame & damn it i won them both [sale seller both in one auction] for $47 [gotta tell the wife lol].

I know yellow turns into blue after the ignition switch. I spliced in the regulator into the yellow where it exits the 8 wire connection under the coil. I was just using the battery voltage to do some testing. So after the engine is installed and magneto wired back into wire harness the relay should do its job and regulate the ac voltage coming from the lighting coil so I don’t get blown bulbs?

If lights are off... the yellow isn't connected to anything... which is fine... voltage has nowhere to go... until you connect the regulator there. Then the regulator has to handle ALL the headlight power from the stator and could overheat and fail.

(Battery acts as a crude "regulator" on the DC side so no need to have the AC one connected with lights off... super rare to blow the DC bulbs with a good battery)

That's why Yamaha connected the regulator after the headlight switch... and be careful... on the US models starting in about 1970 a "blue" out of the key switch could be DC power to run the taillight. Best to follow the Yamaha directions and splice into the blue that powers the gauge illumination bulbs.



P.S. Others have connected regulator to yellow... if you ride with headlight on as required in the US in most places, it won't hurt anything. And may not even if with light off. I've seen them get blistering hot when wired that way.