Dim Headlight on a 1979 Yamaha XS750

Reader Contribution by Keith Fellenstein
Published on July 26, 2011
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Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as your subject. 

Dim headlight on a 1979 Yamaha XS750
Q:
 After a 25-year hiatus from riding, I decided to get back into motorcycling a little while back. I am really enjoying the magazine and your new column. The newer bikes don’t hold the same appeal that vintage Seventies and Eighties bikes do. I purchased a low-mileage 1979 Yamaha XS750, which I have restored and have been riding for the past year. As we are about to get back into prime riding season here in the North, I am continuing to struggle with a problem involving a dim headlight that I am hoping you might help with.  

The problem involves a rather dim output. The headlight is a new halogen type, replaced last year. The light dims at idle, which is something that I would expect due to low output of the alternator at low engine speeds, and does get brighter when RPM rises about 1,500 or so. It still seems really dim, however. The interesting part of this is that when I apply either the front or rear brake enough that the brake light is activated, the brightness of the headlight increases about 30 percent, by my estimate — enough that the light becomes useful! I initially assumed a bad ground that was somehow improved as a result of the brake light circuit, but the schematics do not seem to support this, and I subsequently ran a separate ground wire direct from the headlight to the battery, with no change. As a side note, this same effect is not seen when using other electricals, such as turn signals.  

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