Perhaps the car one you mentioned above was the DKX4 as fitted to Austin 7’s etc and also Ariel Square 4’s. I have a Scott that had one fitted, converted to turn at half engine speed and with two plug leads going to each cap (ie 1 and 3 to one plug, 2 and 4 to the other) due to it turning at half engine speed. It of course therefore advanced the ign twice as much as it should have and ran horribly over about 4000rpm due to over advanced timing. I’m sure that was one contributor to a broken crank in that engine. I have since binned it and put a Lucas magdyno with cable advance operation onto the bike and it runs much better.
As for distributor springs and advance weights they get a seriously hard time at engine speed and wear badly. That causes irratic spark timing (try strobing a Scott and see as you rev it) so the best fix I have found as Brian mentions is just lock it and set the timing at 1/4″ to 5/16″ BTDC.