It seems at the moment this topic is coming up again and again, it makes complete sense but I wandered how many of us understand the rational behind this common modification.
Anyone who has read any of my posts regarding electrics will know that "Wookie keeps banging on about corrosive resistance" and this post will be no exception. It was common on all mass produced vehicles in the 70's and 80's to use barely adequate wiring ( both quality and gauge) Ford Europe were particularly bad for this and fitting upgraded headlight bulbs in a Capri or Escort would often burn out the wiring, light switch and dash loom !!!
Now 40 plus years on age has taken it's toll on that wiring with corroded connections, corrosion between the wire itself and the crimped connector and also those connectors loose spring tension over time.
Below is the igniter for my 82 Ltd and the wiring has age hardened ( you can see the cracks from bending the wire) each one of those cracks will allow moisture to get into the cable and start corroding it) eventually if left there would be green powder deposits around the crack and the circuit would stop working.
The perfect answer for all of us would be a full "re-wire" but that is a big and expensive job which in most cases is uneccessary, a small amount of corrosion will cause volt drops but as long as they are small they won't effect the operation of most electrical consumers ( remember they are designed to work at 12v and when the engine is running the system voltage is around 13.5v so there is some wriggle room)
When it comes to the coil feed this is also largely true ONCE IT IS RUNNING but during starting (especially when cold) the system voltage can be as low as 9.5v to 10v. If you now add in a 1.5v volt drop your coil voltage is down to a little over 8v which is why you have a week spark (a 30% reduction in primary voltage will mean a 30% reduction in secondary voltage at the plugs)
This seems to effect coil voltage the worst as on a lot of these bikes the feed has to go through multiple connectors and switches before it gets to the coil (loosing a little at each on the way) By fitting the by-pass relay you are simply getting back to where it was when new before corrosion took hold and giving the coil full battery voltage
WARNING: if you have electronic ignition be careful, if you have 2 ohm coils and ballast resistors connect the output from your relay before the resistor (not between the resistor and coil) if not you will double the current you put through a 40 plus year old igniter which will problem mean a shopping trip
If you already know all this apologies for boring you but if not, you now know why you are doing what you are doing