Headlight flickering, engine stuttering at speed

[FreddieF]

Hi all,
I've had my 84 Heritage Special for a couple of months and it had been running OK until last weekend when it suddenly started hesitating at highway speed during a long ride. Now it does it after a few minutes of riding. Low speed and idling don't show the problem.

The carbs have been out a few times and everything cleaned - everything is stock. Compression test is OK, cam chain adjusted, valve clearances are good, carbs are synced, no air leaks (unlit propane torch tested). Still no joy. The stuttering seems to get a bit better switching to PRI - but not much better - vacuum? Unrelated?

Last night I went for another test ride and noticed the headlight flickering a bit now and then - aha, inconsistent spark or something. Brushes are just at the point of replacement 7mm - would that cause the problems listed above??

And just how do you reinstall the brushes? Seems you have to turn the brackets upside down and have the wire go around it??? That's strange or I'm doing it wrong.
Thanks for the help!

 

[xjwmx]

Pull the hose loose and turn the valve to prime, and if gas pours out it's not a petcock problem causing the stuttering.

Yes, you have to flip the little plastic piece over the side, to the other side if you have new brushes. Just make it look like the one you're replacing.

The stuttering and the flickering could be unrelated or they could be related. If a loose fuse doesn't show up until a certain amount of vibration it could cause what you're seeing. Theoretically. Check it, but I doubt that would be as consistent as what you're seeing. Could also be a switch or connector or frayed wire.

 

[Classic]

Sounds like you're not getting enough voltage to those coils. Make sure you have a quality, AGM if possible, battery. New brushes are cheap insurance so buy a pair.

With an old bike like that it's a good idea to go through the wiring harness and clean up all of the connectors and grounds. It helps the charging system tremendously.

 

[gggGary]

Yes the brush limit really is that, at 7mm you are having problems, The two issues are related, your bike has the headlight relay powered by the yellow lead off the alternator so when the alternator starts to drop out the headlight will flicker too. The brush wire around the holder is bizarre isn't it?

 

[xjwmx]

^The headlight gets turned on by the yellow wire, but after that it's held on by battery power through a resistor and a diode. That's a safety feature so the light won't go out if the charging system fails. He ought to disconnect a brush with the light on to make sure it stays on, to test the resistor and diode.

 

[FreddieF]

Well, I disconnected a brush and started up the bike - no headlight. Was I supposed to disconnect the brush with the engine running?
With new bushes installed - not too tricky after all - the headlight stayed bright, no flickering or dimming at all, Yay!
The stuttering at speed was MOSTLY gone too - but not completely. Hmmmm . . . more electrical issues you think? I haven't had time to go through the whole harness yet.
Thanks for your help!

 

[pamcopete]

FreddieF,

Yeh, that's the thing that the Yamaha engineers didn't think of, or maybe they did, but when you start the engine with a bad charging system, the headlight will not come on because it is turned on by the safety relay and there is no other way to turn the light on. If the charging system fails after the engine starts then the safety relay in the later models is latched and the headlight stays on. So, the logic probably was that the engineers wanted you to stay put if the charging system was bad at night because you had no way to turn on the headlight, except that if the charging system failed while underway, then they didn't want to turn the headlight off because your heirs might sue them, but if you are on your way home at night and the charging systen fails while underway, you are fine except if you stop to take a leak by the side of the road and shut off the bike to kill the headlight so nobody can see you, you will not have a headlight when you zip up and start the engine....

But if the charging system failed in the day, you would not know that the headlight was not on because the headlight warning light is powered by the headlight circuit.......are you still with me? Not only are you not warned that your headlight is not on, but you are also not warned that your charging system is not working, but without the headlight, the battery will last longer, so maybe you could make it back home, unless you were headed out of town when the charging system failed, then the extended battery life due to the headlight being off would allow you to ride much further before the battery finally died and left you stranded, far, far from home, unless you got lucky and you were stopped by the cops for riding with no headlight...... This is so insane that there must have been a lawyer involved.....

I fixed all this nonsense by installing a relay that is operated by the neutral switch to turn the headlight off when in neutral and on when in gear. I also wired in a diode to illuminate the headlight warning white light if the charging system fails....

 

[xjwmx]

I also wired in a diode to illuminate the headlight warning white light if the charging system fails.

I found this mod on a website that I think is yours, and did it. I think it's a real useful, easy mod.

In my case I deleted the RLU, and didn't need the diodes. The logic behind removing the RLU is as follows. I didn't know whether the RLU was more likely to save me or kill me. In other words, although it appears to work fine I don't know what condition it's actually in at this time or exactly how it works or its failure modes. With it removed, I can always try to get to the filament switch in time with my thumb, on some dark night. I don't know if the RLU isn't going to fail and turn out my light and prevent my thumb from having any effect QED