1972 - CB750 Taillight Issue - Request guidance

Posted by tyler on
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/1972-CB750-Taillight-Issue-Request-guidance-tp4022629.html

Hello guys,

First post from a new member. However I've been a fan of the site for a while now and have found it to be very helpful. With that being said here is the situation:

I own an amber yellow 1972 CB750 that I bought back in mid October 2011. The bike was restored after sitting in a garage in southern California for 10+ years and until now has given me no problems.  As far as I am aware the all the lights and electrical worked correctly expect the high beam; more on that later.

Two weekends ago I was riding up and down the coast when I was pulled over by a pair of what turn out to be very nice motorcycle officers. They told me my rear taillight was not working.  Since both officers owned or own (can't remember) vintage bikes they could sympathize and decided not to give me a fix it ticket. Turns out officers can be nice, who knew?

Nevertheless I immediately headed home, opened up the rear taillight assembly and sure enough found a dead bulb.  A few days later I replaced the rear light bulb according to the Clymer's manuel with a new SAE 1157 compliant bulb and did some checks.  Unfortunately only part of my problem has been solved because it seems that the rear tail light only works uncertain conditions.

If I apply the front, rear or both brakes with the bike running or the key in the on position the rear taillight lights up and operates correctly.  However if I have the bike started or the key in the on position with the headlight on the low setting, the the rear taillight does not come on nor get brighter when either or both brakes are applied.  

I took a reading with my voltmeter from the two contacts in the rear taillight assembly where the bulb screws in and with the headlights off the bike registers 14.2 volts. If the bike is started the reading remains the same at 14.2 volts. However if I switched the headlight on the voltmeter registers 0.29 volts with & without the brakes applied regardless if the bike is started or not.  I can confirm all readings were taken three times to eliminate the possibility of human error and with the help of an assistant.

I can also confirm both turn signals in the front and back operate and flash correctly with the headlight on and off. As I eluded to earlier the front headlight bulb is not stock. I was told that when the bike was restored a new brighter headlight bulb was installed. The one draw back being the fuse would blow if the high beam was used. And so the high beam was disconnected in the front headlight assembly and and electrical wires tapped off. I wrote down the details of the headlight bulb but I cant seem to locate those notes due to a recent move.

To try and fix the problem I've performed these two scenarios:
1) I installed a tested a new & different SAE 1157 rear taillight bulb. My local auto parts store only offered the bulb in packs of two so I conveniently had a spare.  Unfortunately no change to report.
2) Thinking the headlight bulb could be the issue, I disconnected the front headlight cables, switched on the headlight, and checked the rear taillights with & without the brakes applied.  Still the same issue  with the same electrical reading of 0.29 volts.

With that being said, I'm hoping someone has had similar experience and can point me in the right direction.  I'm unable to figure out the problem after searching online, reading through the Clymer's manuel and trouble shooting on my own.

Lastly, thanks for reading my post as I realize its fairly long and probably contains a few grammatical mistakes.

-Tyler