Main fuse blowing

Whats the fuse rating? To test circuits, disconnect the battery (thats already happened as the fuse has blown!) - you are looking for a short to earth so fix one lead to a known good earth and then test each circuit in turn - 0 ohms will be the clue. Check where the wiring runs round the headstock as they often get worn here as you turn the bars. HTH

Just had the same problem, it was an internal short to earth within the harness that after a few fuses blowing became a dead harness as the earth wires burnt out. Fixed it with a new Harness from AN 3 days before leaving for IOM, took 2 days as I took the opportunity to fit a new headlamp harness but also have to wire in the Boyer again and remove some old mods I had made. Quickest way to do it is to leave the old harness in place and starting at the battery work backwards removeing and the remaking each connection swapping from old to new, you can snip sections of the old harness off once all the connections have been made in that area to allow the new harness to take it place. I cleaned all the old connectors and used silicone grease when remaking them. Did the same on a B25 25 years ago with a burnt out harness and its still in place now with no electrical issues in all that time and you can still see the silicone grease.