1971 T500 Electrical Question

[endo_mechanical]

Hi folks, just in the final stages of a T500 restoration, thanks to all for the excellent info on this forum, it has helped me immensely however I have a problem that I need some help with.

So I was ready to start the T500 on the weekend, primed the oil lines, set the timing, points etc and when I was checking spark found that I was getting left and right spark to right spark plug and nothing to the left. I started to check everything and it had me stumped. I had replaced the main loom with a new oem one so started to work my way through the original electrical, there is quite a bit of info on this site regarding this and I kept getting the correct readings. The last thing I checked was the left handlebar switches, I disconnected these at the plug in the headlight shell and found I now had spark at both plugs. At this point I couldn't help myself and had to try and fire her up so connected a small petrol container I use when I have the tank off my bikes and she started after about half a dozen kicks, nice, spent a bit of time, priming oil pump, tuning the carbs, adjusting timing and it was everything I hoped for with my first 2 stroke...noisy & smelly.

So back to the wiring fault, does anyone have any idea what could cause both sparks to one side when the left handlebar loom is plugged in? Regards Richard.

[jabcb]

Wiring diagram is here: viewtopic.php?f=29&t=4743&p=117801&hili ... 00#p117801

During this process, did you disconnect/reconnect any other connectors?

Did you disconnect the coils to take resistance readings? If so, you might have/had a problem there.

Did you charge the battery up? Battery might have been too weak to fire the left plug prior to recharging.

[endo_mechanical]

I disconnected/reconnected just about everything in the process but the only thing that made a difference was the L/H handlebar loom. Checked coil resistance and is within spec. Battery is brand new and fully charged and voltage reads correct.

[jabcb]

If you reconnect the left handlebar loom, are you back to having only one spark?
If that's the case, check the voltage at the coil with the points open & closed.

Do you have a 3 or 4 position ignition switch? (off-on-park or off-on-on/lights-park)
Does your left control light switch have 2 or 3 positions? (L-H or off-L-H)

[endo_mechanical]

OK, I think I have found the problem, seems that the headlight park light power is earthing on to the headlight shell, removed the light from the circuit, had correct spark and she started first kick. I really appreciate the help, sometimes it's good to have a sounding board to stop going in circles, cheers friend. regards Richard

[jabcb]

The problem you found makes sense.

Some of these bikes run on only one cylinder when the voltage is a bit low. So a short that lowered the voltage was a probable cause.