The ignition light will come on for either direction, if the shunt has connected. The reverse light is to tell you your in reverse mode. Both should be out while the engine is running. I was going to ask if it was charging. Sounds like it is.
Spark plug gap - I think, crumbs memory, that a spark has to work harder to jump in the presence of fuel/air mix. its not a charged material, or in a state of excitement, nor is it conductive. So, theory, the density means it cannot leap as far. That is why a wet plug fails to work, as well as offering a shorter route to earth if there is a lot of carbon about. You can get a missfire with the charge nipping across the inner gap of the plug, not out where it can ignite the charge. I believe this is why platenium plugs were offered for in town driving, where deposits could build up and you did not get the chance to blatt that crap out. Living out in a rural area I found these plugs did not work for me but Neville Stevens and other city dwellers used them to better effect, I believe. I have rarely suffered a whiskered plug.
Always worth swapping a plug. They are not really quality items nowadays and I find you have to shuffle to find a good one. Indeed my box of cleaned second hand plugs often supplies a better plug than new! Bless the Trabbi and the recycled Jupiter plug.
