I installed a regulator per tech bulletin M5-084 on my RT3. Thanks MarkT for the suggestion for the pigtail, plug and play is great! After installation I went on a ride and after 50 miles the headlight was still working fine. I noticed it no longer gets brighter when you rev beyond a certain amount, unlike before. When I got back from the ride I noticed my tail light was out, but the stop light worked. I believe on the RT3 the tail light is driven from the magneto (same as headlight) and the stop light is from the battery. Found 1154 Sylvania long life bulbs at the auto store and looked like a close match, but when I installed one it quickly burned out. Do I need the exact 6V21CP(1154) part number 122-84514-30-xx bulb to prevent this, or did adding the regulator protect the headlight, but allow excessive voltage to go to the tail light and if so do I need a regulator there? Two steps forward, one step back, lol. Searched forum, but no hits for this particular issue. Thanks in advance for any suggestions.

The tail light, brake light, horn and turn signals work off the battery. The rest of the lights work from magneto. I have had plenty of RT3's and never had any bike blow out bulbs and never installed a regulator either.

RT3 tail light on a USA model is run off the battery which in effect acts as a rudimentary "voltage regulator"...  if you have a bad or no battery...  or possibly a different type...  (different type of battery may not be able to absorb the excess voltage like the original wet cell in good condition can.)

In my experience, taillights often fail from vibration...  or sometimes a new bulb just pops...  loose bulb socket, cracked fender, loose light...  things like that can increase vibration. 

 

Making sure you have all the correct size bulbs in place, none burned out and a good proper battery in the bike with correct wiring you should be good to go. I suppose adding a regulator for extra insurance in case you do blow a bulb or 2 so the rest of them don't blow can't hurt......... but like i said i never had an issue on any 68-73 Yamaha Enduro blowing bulbs out over the last 20 years working on them and i have worked on many. 

I always made sure (ALL) bulbs were correct size and working, full charged battery in bike and checked the output with engine at around 2500 rpm and up to 8000 rpm before ever turning on the lights and reving the engines up. Last thing i wanted to do was burn out one of these old rare original headlights. Any other bulb can be replaced easy. I always double checked all wiring and tested the rectifier as well. 

So like Mark mentioned 1st questions i would ask is does the bike have a good full charged battery in it. Is all the wiring good and connected properly? Did you test the charging and lighting capacity on the bike? 

 

Thanks all, when I installed the new tail light bulb, I left the tail light lens off and used a mirror to watch it quickly burn out when I changed the switch to position 2. I found that the turn signals glow (not flash) when the brake light is activated and flash slowly at low rpms and faster as the revs build. When I got the bike the battery appeared to hold a charge, but failed a load test so I installed a new battery, but it still loses charge over time, so something in the charging system doesn't appear proper. I tested the rectifier and it flows .575 ohm in one direction and 0 in the other, so that appears ok. Electrical whack a mole, lol. It's a shame, since the bike runs fine except for these gremlins.

Sounds like you've got a short/bad contact somewhere. Just a matter of starting one end and working through to the other, checking and cleaning contacts.

If it only happens when the main switch is in the second position, it might be worth checking the connections on the base of the switch, and also circuit test the switch itself (details are shown alongside the wiring diagram). Post again if this doesn't make sense!

First thing I'd check is battery connections.  Bad battery ground or bad connection or even corrosion in the fuse holder? 

If stator power can't get to the battery through solid connections, then battery can't charge up or act as a reliable "voltage sink" to prevent overvoltage to a bulb. 

You might have just had a defective bulb too.

Not read up to date on this but!!
If its like ours in Ociania "nz" in my case the tail light is from the mag so should have a blue same as instruments & if the reg is into the blue it should go everywhere related to AC, headlight included. If battery powered tail then--i'm lost. I guess tail would be a light brown to be DC powered but take what i say with a grain of salt--unless it makes sense.
Be sure the rectifier is connected as they corrode off terminals. I've seen Regs hooked to the yellow from the mag which would work--i think--but only good if the lights are on all the time. Might overheat it with lights off & high revs. As i said--don't take too much notice, i'll go & read back all the posts later.

Tested the wires to the tail lamp and looked ok. Decided to test the "burned out" bulbs and they looked ok too! Decided to back out my update (unplugged regulator wire), fired up, switched to position 2 and tail light now worked, crazy! Looking in the headlight wiring there appear to be 2 sets of blue wires, the left arrow shows my pigtail and the right arrow shows another blue wire that goes into a black plug. Does it matter which blue wire I tap into? Also found tach/speedo and high beam indicator bulbs are toast, so need to get new ones before I do further testing. How do you remove the high beam indicator without breaking the bulb, does the black case separate? Tried push and twist, but didn't cooperate. Thanks again to everyone for your guidance!

Yes it matters.  One of the blue wires technically should have a stripe?

Background: On the USA models they had to run the taillight off the battery so they added a second blue wire circuit for taillight.

Two checks...  one is with engine OFF and key in the LIGHTS position, one of the two blues coming from the key switch will have battery power.  That's the taillight blue...  one blue comes down from the key switch and connects to a blue that goes into the harness heading towards the rear of the bike... 

The other check is, as I believe the bulletin states, the blue wire to tap into is the blue from the key switch that powers the speedo and tach gauge illumination bulbs (and the hi-lo switch on the handlebar.) It's normally not hard to identify the blue from the switch that connects to the blue gauge bulb wires which is what I do.

I can't see enough in the pic to tell for sure but my guess is the rubber connector goes to the handlebar switch and is the blue you want?

(To add to the confusion, Some US bike models may have used solid blue for the battery operated taillight and blue/white for engine powered headlight...  and then at some point they swapped that around on the later bikes?  Plus stripes can be hard to see and are not always used consistently.  Always best to trace the wires and/or use a meter.)

P.S.  The bad news is the regulator might have fried if it was connected to the battery.