Bike dies when lights turned on.

[Laava]

Ok, here's the dilemma on my SL125 rebuilt bike. When the bike is running and you turn the lights on, it pretty much kills the engine. With the engine off, I can cycle through high and low beam and things seem to operate normally. With the ign off and a volt meter connected I have 6 volts, key on goes to 5.8 v and then with the lights it drops down to about 5.6.v.
If I remove the bulb it all runs fine, and is charging ok. Cycling the light switch makes no difference to anything at this point.
I dont quite understand how the draw from a 6v headlight is enough to effectively stall the engine?
Maybe I need to recheck the rh switchblock which has kill switch and light off/low/high.

[Hoonicorn]

Maybe try a different bulb, less bright (if you're using one of those high-brightness bulbs). Or the voltage regulator/rectifier?

[Laava]

It is the std bulb for these old bikes. In fact there are no high power or special bulbs that I could find. The reg/rec is new and it is charging. Just a mystery that it is dying like this… I am at the point of redoing the entire wiring loom and connections in the hope of finding a cause…it is all brand new with only a couple of exceptions. The loom is brand new…

[jellywrestler]

Ok, here's the dilemma on my SL125 rebuilt bike. When the bike is running and you turn the lights on, it pretty much kills the engine. With the engine off, I can cycle through high and low beam and things seem to operate normally. With the ign off and a volt meter connected I have 6 volts, key on goes to 5.8 v and then with the lights it drops down to about 5.6.v.
If I remove the bulb it all runs fine, and is charging ok. Cycling the light switch makes no difference to anything at this point.
I dont quite understand how the draw from a 6v headlight is enough to effectively stall the engine?
Maybe I need to recheck the rh switchblock which has kill switch and light off/low/high.

P=V X I 6 v bulbs use twice the current to operate. try an led in the tailight that'll release some power usuage. check your earths too.

[pete376403]

What is the voltage at the battery with the motor running at reasonable revs (3-4000) and the lights off? should be a bit higher than 6v (ie a fully charged 12 v battery should be around 13.8 so I guess a 6v battery should be about 6.9)

[neels]

I've come across something similar on one of my bikes in the past.

If the wiring is the same as the diagram I've googled, it has a rectifier but no regulator, and the dynamo designed to get roughly the correct voltage to the battery to charge it. One of the windings (green) is to run the bike, which is a relatively low load. The other winding (red) is switched in when the lights are turned on to provide enough power to run the bike and the lights, if this isn't working there may not be enough volts to keep it running.

I discovered this for the opposite reason, the headlight connection came loose and the extra voltage blew all the other bulbs on the bike, you could test what the voltage is doing in a similar way by disconnecting the headlight bulb but it would risk blowing other stuff that was still connected.

On the diagram I found you could disconnect the headlight (so the bike stays running), and disconnect the white/yellow wire from the white wire (so any extra volts don't blow stuff up), and see if there's AC voltage on the white wire from the dynamo. If there is stick a light bulb between it and ground and see if it's got enough grunt to light it up, if there is then the problem is that it's not getting to where it needs to due to switch or wiring fault.

[Laava]

Thanks guys, gives me some stuff to check…I did fit, and test, a brand new stator and rectifier so I am confident these are up to the task but the wire coding on the rectifier did not mate up to the wiring loom and I had to switch a couple of wires around…

[Laava]

Ok so I thought I would just unplug everything and then plug things back in one by one as I check them. The stator tested fine and I am working on the rectifier. The first thing I checked was the ign switch, which seems to let through .04v when off. It is a two terminal switch, couldn't be simpler and yet it is leaking a tiny amount thru? Anyway, that is not the issue I am looking for…

[jellywrestler]

Ok so I thought I would just unplug everything and then plug things back in one by one as I check them. The stator tested fine and I am working on the rectifier. The first thing I checked was the ign switch, which seems to let through .04v when off. It is a two terminal switch, couldn't be simpler and yet it is leaking a tiny amount thru? Anyway, that is not the issue I am looking for…

probably ghost voltage and not real

[Laava]

probably ghost voltage and not real

Good call.
Actually seems to be a problem with the points/plate. When it is out the points open/close the circuit but when it is in the bike it operates differently. The reason I found it was…also the bike was not running well and I mean how simple is it get a consistent spark right? So I decided to adjust it for advance retard while it was running. Yeah, like you used to under the bonnet of the old holden/falcon/valiant. Climb on in there and wrench the distributor around till it sounded better.
So when I undid one of the pinch bolts on the plate it started to run better so I tightened it up and it died. Cue some poking about with multimeter, trying to work out what the fuck….

[FJRider]

Good call.

Sounds like the issue is similar to a kill switch type issue. It's earthing out somehow ... ??

ie: Bulb in and switched on it dies. Bulb out and switched on it goes.


Hook a similar light of similar size that you can connect directly to the battery. With the motor running .... connect the light directly to positive and negative terminals on the battery. If it doesn't die ... it's not a battery/charging issue.

Sounds like the light is earthing out somewhere in the headlight section of the loom. As above ... like a kill switch.

[neels]

Good call.
Actually seems to be a problem with the points/plate. When it is out the points open/close the circuit but when it is in the bike it operates differently. The reason I found it was…also the bike was not running well and I mean how simple is it get a consistent spark right? So I decided to adjust it for advance retard while it was running. Yeah, like you used to under the bonnet of the old holden/falcon/valiant. Climb on in there and wrench the distributor around till it sounded better.
So when I undid one of the pinch bolts on the plate it started to run better so I tightened it up and it died. Cue some poking about with multimeter, trying to work out what the fuck….

Check resistance between battery negative terminal and various metal bits that should be connected to it. It's starting to sound a bit like there might be an earth missing somewhere, which then could mean volts through the low resistance headlight bulb dragging up what should be a solid 0V somewhere else.

[pete376403]

Check resistance between battery negative terminal and various metal bits that should be connected to it. It's starting to sound a bit like there might be an earth missing somewhere, which then could mean volts through the low resistance headlight bulb dragging up what should be a solid 0V somewhere else.

Or paint under an earthing cable . When the bike has been restored was the frame painted or powder coated? If so was the paint/coating cleaned off to bare metal under the earth connector? Is there an earth strap between frame and engine? (really only matters if the engine is rubber mounted but doesnt hurt to be sure)

[Laava]

Check resistance between battery negative terminal and various metal bits that should be connected to it. It's starting to sound a bit like there might be an earth missing somewhere, which then could mean volts through the low resistance headlight bulb dragging up what should be a solid 0V somewhere else.

Or paint under an earthing cable . When the bike has been restored was the frame painted or powder coated? If so was the paint/coating cleaned off to bare metal under the earth connector? Is there an earth strap between frame and engine? (really only matters if the engine is rubber mounted but doesnt hurt to be sure)

Yep I have gone overboard with earth straps as I have had this situation before, I don't have the time now for a test run. Not helped by the fact my brand new fuel tap is leaking and I would rather fix it before I put the tank back on…
thanks for your input everybody…

[Laava]

Right so I have removed, checked and repaired the whole wiring loom, sorted a random earthing problem on the points and it has made zero difference. The switchgear is working as it should according to my tester and I have gotten nowhere. Hard to believe I was riding this bike a few weeks ago now…
which brings me back to my previous problem, getting it to even run. It started up on idle and immediately dies at the slightest throttle. If you baby it to get some revs up, then you can give it throttle and it revs up but kinda crackly and not happy. Still pretty much dies when you turn the lights on unless you have the revs up.
So it is going to be at the bike show in non running condition and then I may even sell it like that. I never thought it would be so ongoingly difficult to get such a basic machine to run properly.
Basically to get it to go I seem to have to pull the spark plug and clean and dry it to have a hope. I am over it now sadly. I was hoping to keep it for the odd ride but it just isnt reliable enough for that. And I have Ducatis so that is saying something!…