Electrical problem??

Hello,
Have a 67 sx150, very original bike still have the original plug cap. I can start the bike and it revs but when I turn the lights on it drains the power. If I release the throttle it will idle. I've changed the HT lead & cap. Added an earth wire to the frame that was missing. All the lights and horn work.
Could it be timing?
Or should I try a different coil first? It's still running on points?
Anyone experienced something similar?
Thanks Lee
Have a 67 sx150, very original bike still have the original plug cap. I can start the bike and it revs but when I turn the lights on it drains the power. If I release the throttle it will idle. I've changed the HT lead & cap. Added an earth wire to the frame that was missing. All the lights and horn work.
Could it be timing?
Or should I try a different coil first? It's still running on points?
Anyone experienced something similar?
Thanks Lee