Headlight keeps dimming

I have a 79 KZ650. Just bought it this year and everything was working great, but one night I fired it up and the low beam was out.

I replaced the headlight and right after this I started having my problem. Whenever I get the RPMs upto approx 3,500 - 3,600 the headlight will actually go out. As I approx this range within +/- 200 RPMs the headlight will start dimming and if I get over this range it comes back on fine. I have no problems with the headlight at any other RPM.

The dimming is only with the headlight and none of the other lights. Also I tested this with the High beam and the high beam doesn't have this issue.

I didn't have this problem until after the headlight burned out and I replaced it.

I though it might be loose wiring, but I've wiggled all the wires in the headlight housing and nothing happens.

Any ideas?

Thanks.

Just a guess, it may be possible the low beam filament is bad, and when the vibrations get in that rpm range it cuts out? Try a new bulb, maybe?

Or you can hook up a meter to the low beam wire and measure the voltage as you go up in rpms?

I had a similar symptom.

1) Check that the power rating of the New head light is the same as the old. If the new one is higher it is drawing more current and heating up a contact that was OK for the older and lower rating head light. Or, you may have a battery or charging limitation or fault.

2) Check the contacts in your head light assembly. Look for poor contacts, signs of over heating in wires and connector shells and especially check for good ground. An easy test is to run a temporary Battery and Ground directly from the Battery to the head light (preferably through an in line 10 Amp fuse) and see if the symptom disappears. If it does disappear, then troubleshoot or replace the wiring. If they do not disappear, then troubleshoot the charging system. You will want to have a repair manual at this point.

3) If you have the problem I had, then checking if the symptom follows your Hi/Lo switch setting or movement will help. If it follows the switch setting or movement, then clean out switch (watch out for spring loaded parts flying out) and or use a relay to allow the high current to bypass any poor or intermittent contacts in your switch gear, associated wiring and contacts. Let me know, I have a wiring diagram for a Head Light Relay mod. that I had to make and install. It was cheaper than new switch gear for me.

Best of success!

Thanks for the replies. Turns out it was a bad bulb. When I took the headlight out of the housing it wouldn't dim, but if left in it would.

Got a replacement bulb and installed it problem went away.

Good stuff! Thanks for leting us know what the solution was. Great call by OKC_Kent!

Could you add your year, model and location to your signature... its considered very friendly.

Best of success!